diff --git a/ouexam.dtx b/ouexam.dtx
index 986c255..15ff872 100644
--- a/ouexam.dtx
+++ b/ouexam.dtx
@@ -36,7 +36,7 @@
 %
 % \fi
 %
-%% \CheckSum{889}
+%% \CheckSum{912}
 %%
 %% \CharacterTable
 %%  {Upper-case    \A\B\C\D\E\F\G\H\I\J\K\L\M\N\O\P\Q\R\S\T\U\V\W\X\Y\Z
@@ -111,7 +111,7 @@
 % Version 2.0 of \textsf{ouexam} introduced several major changes from earlier
 % versions. Consequently, documents created using an earlier version of
 % \textsf{ouexam} are \emph{incompatible} with version 2.0 or later. Versions 2.0
-% and greater of \textsf{ouexam} will detect attempts to process version 1.x
+% and greater of \textsf{ouexam} will detect attempts to process a version 1.x
 % document and display an error message.
 % 
 % The older version (1.2) is still available and can be downloaded from the same
@@ -428,6 +428,9 @@
 % 	depending on how full the last line of the question is.
 % \end{itemize}
 % 
+% \changes{2.3.1}{2013/05/31}{NJS Enforced proper question environment nesting.}
+% The various \textsf{question} environments must be correctly nested in order for the features described above to work correctly, i.e., \textsf{question} at the top level, followed by \textsf{subquestion} and \textsf{subsubquestion}. Any variation to this nesting order will cause an error.
+% 
 % 
 % \changes{2.2}{2010/04/26}{NJS Added time allocation macros.}
 % \subsection{Time allocations}
@@ -1350,6 +1353,16 @@
 % \end{macro}
 % \end{macro}
 % 
+% \begin{macro}{qdepth}
+% \changes{2.3.1}{2013/05/31}{NJS New \texttt{qdepth} counter.}
+% This counter is incremented every time a \textsf{question}, \textsf{subquestion} or \textsf{subsubquestion} environment is opened, and decremented every time these environments close. It measures the nesting depth of questions, and is used to check that we don't have things like a \texttt{subquestion} that isn't enclosed by a \texttt{question}, or a \texttt{question} nested inside another \texttt{question}. Note that this counter does \emph{not} reset inside any other environment.
+% 
+%    \begin{macrocode}
+\newcounter{qdepth}
+\setcounter{qdepth}{0}
+%    \end{macrocode}
+% \end{macro}
+% 
 % At the end of the document, check that the running total of marks for the
 % examination matches what we were expecting:
 % 
@@ -1387,6 +1400,8 @@
 % \changes{2.0}{2001/05/15}{NJS Total marks for a question are now printed only
 % if the question has no sub-questions.}
 % \begin{environment}{question}
+% \changes{2.3.1}{2013/05/31}{NJS Check question depth when opening the
+% environment.}
 % The \textsf{question} environment specifies a top-level question, and produces
 % questions numbered in the form ``1.'', ``2.'', etc. It has one mandatory
 % argument, the number of marks allocated to the question, which is used to
@@ -1402,6 +1417,15 @@
   \else\setcounter{qexpected}{#1}\fi%
 %    \end{macrocode}
 % 
+% Check that the environment is being opened at the correct depth. The \textsf{question} environment should normally only be opened at depth 0:
+%    \begin{macrocode}
+  \ifnum\theqdepth=0%
+  \else\ClassError{ouexam}{question environment nested inside another %
+        question, subquestion or subsubquestion environment}%
+  \fi%
+  \addtocounter{qdepth}{1}%
+%    \end{macrocode}
+% 
 % For each top-level question, we increment the |question| counter, which also
 % resets the |hassubs| and |qrunning| counters to zero:
 %    \begin{macrocode}
@@ -1463,6 +1487,12 @@
 %    \begin{macrocode}
     \setcounter{lastexpected}{\value{qexpected}}%
   \end{list}%
+%    \end{macrocode}
+% 
+% 	\item |qdepth| is decremented.
+% 
+%    \begin{macrocode}
+  \addtocounter{qdepth}{-1}%
 }
 %    \end{macrocode}
 % 
@@ -1474,6 +1504,8 @@
 % redefinition of \textsf{enumerate}.}
 % \changes{2.0}{2001/05/13}{NJS Added penalty handling to ensure correct placement
 % of marks in output.}
+% \changes{2.3.1}{2013/05/31}{NJS Check question depth when opening the
+% environment.}
 % The \textsf{subquestion} environment is the analogue of \textsf{question} for
 % sub-questions. On initialisation, it sets |subqexpected| to the value of the
 % mandatory argument (defaulting to zero if the argument is empty), increments
@@ -1490,6 +1522,15 @@
   \setcounter{subqexpected}{#1}%
   \addtocounter{hassubs}{1}%
   \refstepcounter{subquestion}%
+%    \end{macrocode}
+% The \textsf{subquestion} environment should normally only be opened at depth 1:
+%    \begin{macrocode}
+  \ifnum\theqdepth=1%
+  \else\ClassError{ouexam}{subquestion environment either outside a %
+        question environment, or nested inside another subquestion or %
+        subsubquestion environment}%
+  \fi%
+  \addtocounter{qdepth}{1}%
   \begin{list}{\labelsubquestion}{\settowidth{\labelwidth}{(m)}}%
     \item \ignorespaces%
 }{%
@@ -1530,6 +1571,7 @@
 %    \begin{macrocode}
     \setcounter{lastexpected}{\value{subqexpected}}%
   \end{list}%
+  \addtocounter{qdepth}{-1}%
 }
 %    \end{macrocode}
 % \end{environment}
@@ -1539,6 +1581,8 @@
 % redefinition of \textsf{enumerate}.}
 % \changes{2.0}{2001/05/13}{NJS Added penalty handling to ensure correct placement
 % of marks in output.}
+% \changes{2.3.1}{2013/05/31}{NJS Check question depth when opening the
+% environment.}
 % This is similar to both \textsf{question} and \textsf{subquestion}, and
 % generates an appropriately numbered sub-sub-question (``(i)'', ``(ii)'', etc.).
 %    \begin{macrocode}
@@ -1552,6 +1596,15 @@
 %    \begin{macrocode}
   \addtocounter{hassubsubs}{1}%
   \refstepcounter{subsubquestion}%
+%    \end{macrocode}
+% The \textsf{subsubquestion} environment should normally only be opened at depth 2:
+%    \begin{macrocode}
+  \ifnum\theqdepth=2%
+  \else\ClassError{ouexam}{subsubquestion environment either outside a %
+        subquestion environment, or nested inside another subsubquestion %
+        environment}%
+  \fi%
+  \addtocounter{qdepth}{1}%
   \begin{list}{\labelsubsubquestion}{\settowidth{\labelwidth}{(viii)}}%
     \item \ignorespaces%
 }{%
@@ -1573,14 +1626,11 @@
                               \else marks\fi)%
     \parfillskip=0pt \finalhyphendemerits=0 \par%
   \end{list}%
+  \addtocounter{qdepth}{-1}%
 }
 %    \end{macrocode}
 % \end{environment}
 % 
-% Note that none of these three environments check whether they are correctly
-% nested (i.e., \textsf{subsubquestion} within \textsf{subquestion} within
-% \textsf{question}).
-% 
 % 
 % \subsection{Draft examination printing}
 %