diff --git a/luamml-patches-amsmath.sty b/luamml-patches-amsmath.sty
index 7e8ced2..f8933c4 100644
--- a/luamml-patches-amsmath.sty
+++ b/luamml-patches-amsmath.sty
@@ -42,7 +42,7 @@
           \m@th
           \displaystyle
           {##}
-          \luamml_flag_save:Nn \displaystyle {mtd}
+          \luamml_flag_save:nNn {} \displaystyle {mtd}
         $
         \__luamml_amsmath_add_last_to_row:
         \tabskip \z@skip
@@ -54,7 +54,7 @@
             {}
             ##
           }
-          \luamml_flag_save:Nn \displaystyle {mtd}
+          \luamml_flag_save:nNn {} \displaystyle {mtd}
         $
         \__luamml_amsmath_add_last_to_row:
         \hfil
@@ -85,7 +85,7 @@
           \m@th
           \displaystyle
           ##
-          \luamml_flag_save:Nn \displaystyle {mtd}
+          \luamml_flag_save:nNn {} \displaystyle {mtd}
         $
         \__luamml_amsmath_add_last_to_row:
         \hfil
@@ -122,7 +122,7 @@
           \m@th
           \displaystyle
           {##}
-          \luamml_flag_save:Nn \displaystyle {mtd}
+          \luamml_flag_save:nNn {} \displaystyle {mtd}
         $
       }
       \__luamml_amsmath_add_box_to_row:
@@ -180,7 +180,7 @@
         \ifmeasuring@
           \luamml_flag_ignore:
         \else
-          \luamml_flag_save:Nn \displaystyle {mtd}
+          \luamml_flag_save:nNn {} \displaystyle {mtd}
         \fi
       $
     }
@@ -204,7 +204,7 @@
       \ifmeasuring@
         \luamml_flag_ignore:
       \else
-        \luamml_flag_save:Nn \displaystyle {mtd}
+        \luamml_flag_save:nNn {} \displaystyle {mtd}
       \fi
       $
     }
@@ -298,14 +298,14 @@
 % Less luckily, \endmultline@math sometimes get overwritten for the last line.
 % But that isn't a problem since we want special behavior there anyway.
 \cs_set:Npn \endmultline@math {
-  \luamml_flag_save:Nn \displaystyle {mtd}
+  \luamml_flag_save:nNn {} \displaystyle {mtd}
   $
   \__luamml_amsmath_add_last_to_row:
 }
 
 \cs_set:Npn \rendmultline@ {
     \iftag@
-      \luamml_flag_save:Nn \displaystyle {mtd}
+      \luamml_flag_save:nNn {} \displaystyle {mtd}
       $
       \__luamml_amsmath_add_last_to_row:
       \let \endmultline@math \relax
@@ -370,7 +370,7 @@
         \m@th
         \scriptstyle
         ##
-        \luamml_flag_save:n {mtd} % No \scriptsize here since we want to add the mstyle nodes
+        \luamml_flag_save:nn {} {mtd} % No \scriptsize here since we want to add the mstyle nodes
         $
         \__luamml_amsmath_add_last_to_row:
         \hfil
@@ -381,7 +381,7 @@
         \m@th
         \scriptstyle
         ##
-        \luamml_flag_save:n {mtd} % No \scriptsize here since we want to add the mstyle nodes
+        \luamml_flag_save:nn {} {mtd} % No \scriptsize here since we want to add the mstyle nodes
         $
         \__luamml_amsmath_add_last_to_row:
         \hfil
diff --git a/luamml-patches-array.sty b/luamml-patches-array.sty
index dc15a40..7635323 100644
--- a/luamml-patches-array.sty
+++ b/luamml-patches-array.sty
@@ -18,7 +18,7 @@
       } {
         \__luamml_array_init_col:
         \insert@column
-        \luamml_flag_save:n {mtd}
+        \luamml_flag_save:nn {} {mtd}
         \d@llarend
         \__luamml_array_finalize_col:w 0~
       }
@@ -33,7 +33,7 @@
       } {
         \__luamml_array_init_col:
         \insert@column
-        \luamml_flag_save:n {mtd}
+        \luamml_flag_save:nn {} {mtd}
         \d@llarend
         \__luamml_array_finalize_col:w 1~
       }
@@ -49,7 +49,7 @@
       } {
         \__luamml_array_init_col:
         \insert@column
-        \luamml_flag_save:n {mtd}
+        \luamml_flag_save:nn {} {mtd}
         \d@llarend
         \__luamml_array_finalize_col:w 2~
       }
diff --git a/luamml-patches-kernel.sty b/luamml-patches-kernel.sty
index bec471b..1ae497a 100644
--- a/luamml-patches-kernel.sty
+++ b/luamml-patches-kernel.sty
@@ -9,7 +9,7 @@
     \m@th
     #1
     {#2}
-    \luamml_flag_save:Nn #1 {mphantom}
+    \luamml_flag_save:nNn {} #1 {mphantom}
     $
   }
   \finph@nt
diff --git a/luamml-tex.lua b/luamml-tex.lua
index d7ac64e..a9be147 100644
--- a/luamml-tex.lua
+++ b/luamml-tex.lua
@@ -9,6 +9,7 @@ local write_xml = require'luamml-xmlwriter'
 local write_struct = require'luamml-structelemwriter'
 
 local filename_token = token.create'l__luamml_filename_tl'
+local label_token = token.create'l__luamml_label_tl'
 
 local properties = node.get_properties_table()
 local mmode, hmode, vmode do
@@ -70,6 +71,8 @@ local mlist_result
 local undefined_cmd = token.command_id'undefined_cs'
 local call_cmd = token.command_id'call'
 
+local labelled_mathml = {}
+
 local function save_result(xml, display, structelem)
   mlist_result = make_root(xml, display and 0 or 2)
   token.put_next(filename_token)
@@ -121,6 +124,18 @@ luatexbase.add_to_callback('pre_mlist_to_hlist_filter', function(mlist, style)
       properties[startmath] = props
     end
     props.saved_mathml_table, props.saved_mathml_core = xml, core
+    token.put_next(label_token)
+    local label = token.scan_argument()
+    if label ~= '' then
+      if labelled_mathml[label] then
+        tex.error('MathML Label already in use', {
+            'A MathML expression has a label which is already used by another \z
+             formula. If you do not want to label this formula with a unique \z
+             label, set a empty label instead.'})
+      else
+        labelled_mathml[label] = xml
+      end
+    end
     if flag & 10 == 8 then
       write_struct(xml, true) -- This modifies xml in-place to reference the struture element
     end
@@ -148,4 +163,5 @@ require'luamml-tex-annotate'
 
 return {
   save_result = save_result,
+  labelled = labelled_mathml,
 }
diff --git a/luamml.dtx b/luamml.dtx
index 8faa720..99038fc 100644
--- a/luamml.dtx
+++ b/luamml.dtx
@@ -88,6 +88,7 @@
 %<luatex>\tl_new:N \l__luamml_filename_tl
 \tl_new:N \l__luamml_root_tl
 \tl_set:Nn \l__luamml_root_tl { mrow }
+\tl_new:N \l__luamml_label_tl
 %<pdftex>\int_new:N \g__luamml_formula_id_int
 %<luatex>\int_new:N \tracingmathml
 %    \end{macrocode}
@@ -110,6 +111,7 @@
 %   \cs{luamml_flag_save:}.
 %    \begin{macrocode}
 \cs_new_protected:Npn \luamml_flag_process: {
+  \tl_set:Nn \l__luamml_label_tl {}
   \int_set:Nn \l__luamml_flag_int { 3 }
 }
 %    \end{macrocode}
@@ -128,33 +130,37 @@
 }
 % \end{macro}
 %
-% \begin{macro}{\luamml_flag_save:,
-%               \luamml_flag_save:N,
-%               \luamml_flag_save:n,
-%               \luamml_flag_save:Nn}
+% \begin{macro}{\luamml_flag_save:n,
+%               \luamml_flag_save:nN,
+%               \luamml_flag_save:nn,
+%               \luamml_flag_save:nNn}
 %   Convert the current formula but only save it's representation in the math
 %   node without emitting it as a complete formula. This is useful when the
 %   expression forms part of a bigger formula and will be intergrated into it's
 %   MathML tables later by special code.
-%   It optinally accepts two parameters: One math style command
+%   It optinally accepts three parameters: A label, one math style command
 %   (\cs{displaystyle}, \cs{textstyle}, etc.) which is the implicit math style
 %   (so the style which the surrounding code expects this style to have) and a
 %   name for the root element (defaults to \texttt{mrow}).
 %   If the root element name is \texttt{mrow}, it will get suppressed in some
 %   cases.
 %    \begin{macrocode}
-\cs_new_protected:Npn \luamml_flag_save: {
+\cs_new_protected:Npn \luamml_flag_save:n #1 {
+  \tl_set:Nn \l__luamml_label_tl {#1}
   \int_set:Nn \l__luamml_flag_int { \__luamml_maybe_structelem: 1 }
 }
-\cs_new_protected:Npn \luamml_flag_save:N #1 {
-  \int_set:Nn \l__luamml_flag_int { \__luamml_maybe_structelem: 17 + 32 * #1 }
+\cs_new_protected:Npn \luamml_flag_save:nN #1#2 {
+  \tl_set:Nn \l__luamml_label_tl {#1}
+  \int_set:Nn \l__luamml_flag_int { \__luamml_maybe_structelem: 17 + 32 * #2 }
 }
-\cs_new_protected:Npn \luamml_flag_save:n {
+\cs_new_protected:Npn \luamml_flag_save:nn #1 {
+  \tl_set:Nn \l__luamml_label_tl {#1}
   \int_set:Nn \l__luamml_flag_int { \__luamml_maybe_structelem: 5 }
   \tl_set:Nn \l__luamml_root_tl
 }
-\cs_new_protected:Npn \luamml_flag_save:Nn #1 {
-  \int_set:Nn \l__luamml_flag_int { \__luamml_maybe_structelem: 21 + 32 * #1 }
+\cs_new_protected:Npn \luamml_flag_save:nNn #1#2 {
+  \tl_set:Nn \l__luamml_label_tl {#1}
+  \int_set:Nn \l__luamml_flag_int { \__luamml_maybe_structelem: 21 + 32 * #2 }
   \tl_set:Nn \l__luamml_root_tl
 }
 %    \end{macrocode}
@@ -176,6 +182,7 @@
 %    \begin{macrocode}
 %<*luatex>
 \cs_new_protected:Npn \luamml_flag_structelem: {
+  \tl_set:Nn \l__luamml_label_tl {}
   \int_set:Nn \l__luamml_flag_int { 11 }
 }
 %</luatex>
@@ -350,6 +357,8 @@
     \int_use:N \l__luamml_flag_int
     :
     \l__luamml_root_tl
+    :
+    \l__luamml_label_tl
   }
   \__luamml_pdf_showlists:
   \iow_now:Nx \l__luamml_pdf_stream {
@@ -359,16 +368,6 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}{\luamml_pdf_last_formula:}
-% A unique identifier for the last formula written with \cs{luamml_pdf_write:}.
-% Some form of this will probably be added to the Lua\TeX insterface at some point.
-%    \begin{macrocode}
-\cs_new:Npn \luamml_pdf_last_formula: {
-  \int_use:N \g__luamml_formula_id_int
-}
-%    \end{macrocode}
-% \end{macro}
-%
 %    \begin{macrocode}
 %</pdftex>
 %    \end{macrocode}
diff --git a/pdfmml-logreader.lua b/pdfmml-logreader.lua
index a17f604..19d410e 100644
--- a/pdfmml-logreader.lua
+++ b/pdfmml-logreader.lua
@@ -8,7 +8,7 @@ local math_lists_block = l.Ct('### ' * l.Cg(l.C'display' * ' ', 'display')^-1 *
                        * non_final_list_block)^1
 local generic_list_block = '### ' * (line - 'current page:') * non_final_list_block
 local luamml_block = l.Cg('LUAMML_FORMULA_BEGIN:' * id * ':' * l.Ct(
-   l.Cg(id, 'flag') * ':' * l.Cg((1-l.P'\n')^0, 'tag') * l.P'\n'^1
+   l.Cg(id, 'flag') * ':' * l.Cg((1-l.S':\n')^0, 'tag') * ':' * l.Cg((1-l.P'\n')^1, 'label')^-1 * l.P'\n'^1
    
  * (math_lists_block + generic_list_block/0)^0
  * (line - 'LUAMML_FORMULA_END\n')^0
diff --git a/pdfmml.lua b/pdfmml.lua
index 116a617..05d16da 100755
--- a/pdfmml.lua
+++ b/pdfmml.lua
@@ -67,7 +67,7 @@ end
 --   1: Only save
 --   3: Generate normally
 for i, block in ipairs(parsed.groups) do
-  local flag, tag = block.flag, block.tag
+  local flag, tag, label = block.flag, block.tag, block.label
   block = block[1]
   if flag & 3 ~= 0 then
     local style = block.display and 0 or 2
@@ -84,8 +84,11 @@ for i, block in ipairs(parsed.groups) do
         xml = {[0] = tag, xml}
       end
     end
-    if style == 2 and flag & 1 == 1 then
-      parsed.mathml[i] = xml
+    if style == 2 and flag & 1 == 1 and label ~= '' then
+      if parsed.mathml[label] then
+        error'Invalid label reuse'
+      end
+      parsed.mathml[label] = xml
     end
   end
 end