diff --git a/autoload/vimtex/syntax/p/amsmath.vim b/autoload/vimtex/syntax/p/amsmath.vim
index c9e1d784a8..394acac886 100644
--- a/autoload/vimtex/syntax/p/amsmath.vim
+++ b/autoload/vimtex/syntax/p/amsmath.vim
@@ -31,17 +31,45 @@ function! vimtex#syntax#p#amsmath#load(cfg) abort " {{{1
   syntax match texMathCmdEnv contained contains=texCmdMathEnv                                            "\\end{x\?alignat\*\?}"
   syntax match texMathCmdEnv contained contains=texCmdMathEnv                                            "\\end{xxalignat}"
 
+  " numberwithin
+  syntax match texCmdNumberWithin "\\numberwithin\>"
+        \ nextgroup=texNumberWithinArg1 skipwhite skipnl
+  call vimtex#syntax#core#new_arg('texNumberWithinArg1', {
+        \ 'next': 'texNumberWithinArg2',
+        \ 'contains': 'TOP,@Spell'
+        \})
+  call vimtex#syntax#core#new_arg('texNumberWithinArg2', {
+        \ 'contains': 'TOP,@Spell'
+        \})
+
+  " subjclass
+  syntax match texCmdSubjClass "\\subjclass\>"
+        \ nextgroup=texSubjClassOpt,texSubjClassArg skipwhite skipnl
+  call vimtex#syntax#core#new_opt('texSubjClassOpt', {
+        \ 'next': 'texSubjClassArg',
+        \ 'contains': 'TOP,@Spell'
+        \})
+  call vimtex#syntax#core#new_arg('texSubjClassArg', {
+        \ 'contains': 'TOP,@Spell'
+        \})
+
   " DeclareMathOperator
   syntax match texCmdDeclmathoper nextgroup=texDeclmathoperArgName skipwhite skipnl "\\DeclareMathOperator\>\*\?"
   call vimtex#syntax#core#new_arg('texDeclmathoperArgName', {
         \ 'next': 'texDeclmathoperArgBody',
         \ 'contains': ''
         \})
-  call vimtex#syntax#core#new_arg('texDeclmathoperArgBody')
+  call vimtex#syntax#core#new_arg('texDeclmathoperArgBody', {'contains': 'TOP,@Spell'})
+
+  " operatorname
+  syntax match texCmdOpname nextgroup=texOpnameArg skipwhite skipnl "\\operatorname\>"
+  call vimtex#syntax#core#new_arg('texOpnameArg', {
+        \ 'contains': 'TOP,@Spell'
+        \})
 
   " \tag{label} or \tag*{label}
   syntax match texMathCmd "\\tag\>\*\?" contained nextgroup=texMathTagArg
-  call vimtex#syntax#core#new_arg('texMathTagArg')
+  call vimtex#syntax#core#new_arg('texMathTagArg', {'contains': 'TOP,@Spell'})
 
   " Conceal the command and delims of "\operatorname{ ... }"
   if g:vimtex_syntax_conceal.math_delimiters
@@ -52,8 +80,17 @@ function! vimtex#syntax#p#amsmath#load(cfg) abort " {{{1
   endif
 
   highlight def link texCmdDeclmathoper     texCmdNew
+  highlight def link texCmdNumberWithin     texCmd
+  highlight def link texCmdOpName           texCmd
+  highlight def link texCmdSubjClass        texCmd
   highlight def link texDeclmathoperArgName texArgNew
+  highlight def link texDeclmathoperArgBody texMathZone
   highlight def link texMathConcealedArg    texMathTextArg
+  highlight def link texNumberWithinArg1    texArg
+  highlight def link texNumberWithinArg2    texArg
+  highlight def link texOpnameArg           texMathZone
+  highlight def link texSubjClassArg        texArg
+  highlight def link texSubjClassOpt        texOpt
 endfunction
 
 " }}}1
diff --git a/autoload/vimtex/syntax/p/amsthm.vim b/autoload/vimtex/syntax/p/amsthm.vim
index def523f5c8..fbb30f03db 100644
--- a/autoload/vimtex/syntax/p/amsthm.vim
+++ b/autoload/vimtex/syntax/p/amsthm.vim
@@ -15,7 +15,15 @@ function! vimtex#syntax#p#amsthm#load(cfg) abort " {{{1
         \ 'contains': 'TOP,@NoSpell'
         \})
 
+  syntax match texCmdThmStyle "\\theoremstyle\>"
+        \ nextgroup=texThmStyleArg skipwhite skipnl
+  call vimtex#syntax#core#new_arg('texThmStyleArg', {
+        \ 'contains': 'TOP,@Spell'
+        \})
+
+  highlight def link texCmdThmStyle texCmd
   highlight def link texProofEnvOpt texEnvOpt
+  highlight def link texThmStyleArg texArg
 endfunction
 
 " }}}1
diff --git a/test/test-syntax/test-amsmath.tex b/test/test-syntax/test-amsmath.tex
index e2a79dd64c..04235533bd 100644
--- a/test/test-syntax/test-amsmath.tex
+++ b/test/test-syntax/test-amsmath.tex
@@ -1,6 +1,13 @@
-\documentclass{article}
+\documentclass{amsart}
 \usepackage{amsmath}
 
+\DeclareMathOperator{\GL}{GL}
+\newcommand{\SL}{\operatorname{SL}}
+
+\numberwithin{equation}{section}
+
+\subjclass[2010]{Primary: 11F67}
+
 \begin{document}
 
 \begin{align}
diff --git a/test/test-syntax/test-amsmath.vim b/test/test-syntax/test-amsmath.vim
index 4821e708a9..482146b290 100644
--- a/test/test-syntax/test-amsmath.vim
+++ b/test/test-syntax/test-amsmath.vim
@@ -2,7 +2,23 @@ source common.vim
 
 silent edit test-amsmath.tex
 
+set spell
+
 if empty($INMAKE) | finish | endif
 
+call assert_true(vimtex#syntax#in('texCmdDeclmathoper', 4, 1))
+call assert_true(vimtex#syntax#in('texDeclmathoperArgName', 4, 22))
+call assert_true(vimtex#syntax#in('texDeclmathoperArgBody', 4, 27))
+
+call assert_true(vimtex#syntax#in('texCmdOpname', 5, 18))
+call assert_true(vimtex#syntax#in('texOpnameArg', 5, 32))
+
+call assert_true(vimtex#syntax#in('texCmdNumberWithin', 7, 1))
+call assert_true(vimtex#syntax#in('texNumberWithinArg1', 7, 15))
+call assert_true(vimtex#syntax#in('texNumberWithinArg2', 7, 25))
+
+call assert_true(vimtex#syntax#in('texCmdSubjClass', 9, 1))
+call assert_true(vimtex#syntax#in('texSubjClassOpt', 9, 12))
+call assert_true(vimtex#syntax#in('texSubjClassArg', 9, 18))
 
-quit!
+call vimtex#test#finished()
diff --git a/test/test-syntax/test-amsthm.tex b/test/test-syntax/test-amsthm.tex
index c8e9ed3323..264f628a24 100644
--- a/test/test-syntax/test-amsthm.tex
+++ b/test/test-syntax/test-amsthm.tex
@@ -1,6 +1,7 @@
 \documentclass{article}
 \usepackage{amsthm}
 
+\theoremstyle{plain}
 \newtheorem{thm}{Theorem}[section]
 \newtheorem{prp}[thm]{Proposition}
 \newtheorem{lemma}{Lemma}
@@ -19,4 +20,8 @@
   A lemma here.
 \end{lemma}
 
+\begin{proof}[Proof of theorem abc]
+    Proof goes here.
+\end{proof}
+
 \end{document}
diff --git a/test/test-syntax/test-amsthm.vim b/test/test-syntax/test-amsthm.vim
index a1c0048e14..6b5047ed0f 100644
--- a/test/test-syntax/test-amsthm.vim
+++ b/test/test-syntax/test-amsthm.vim
@@ -6,15 +6,20 @@ set spell
 
 if empty($INMAKE) | finish | endif
 
-call assert_true(vimtex#syntax#in('texNewthmOptNumberby', 4, 32))
-call assert_true(vimtex#syntax#in('texNewthmOptCounter', 5, 19))
+call assert_true(vimtex#syntax#in('texCmdThmStyle', 4, 1))
+call assert_true(vimtex#syntax#in('texThmStyleArg', 4, 15))
+call assert_true(vimtex#syntax#in('texNewthmOptNumberby', 5, 32))
+call assert_true(vimtex#syntax#in('texNewthmOptCounter', 6, 19))
 
-call assert_true(vimtex#syntax#in('texTheoremEnvBgn', 10, 1))
-call assert_true(vimtex#syntax#in('texTheoremEnvBgn', 14, 1))
-call assert_true(vimtex#syntax#in('texTheoremEnvBgn', 18, 1))
+call assert_true(vimtex#syntax#in('texProofEnvBgn', 23, 1))
+call assert_true(vimtex#syntax#in('texProofEnvOpt', 23, 15))
 
-call assert_true(vimtex#syntax#in('texTheoremEnvOpt', 10, 36))
-call assert_true(vimtex#syntax#in('texCmdRefConcealed', 10, 42))
-call assert_true(vimtex#syntax#in('texRefConcealedArg', 10, 47))
+call assert_true(vimtex#syntax#in('texTheoremEnvBgn', 11, 1))
+call assert_true(vimtex#syntax#in('texTheoremEnvBgn', 15, 1))
+call assert_true(vimtex#syntax#in('texTheoremEnvBgn', 19, 1))
+
+call assert_true(vimtex#syntax#in('texTheoremEnvOpt', 11, 36))
+call assert_true(vimtex#syntax#in('texCmdRefConcealed', 11, 42))
+call assert_true(vimtex#syntax#in('texRefConcealedArg', 11, 47))
 
 call vimtex#test#finished()