Hello, I did not try to analyze how the following macros work (they are too complex), but they somehow fail in mkiv. (I don't need the "F" kerning, so that portion of code may go away, but the macro itself is very very handy.) All I want is to have a macro that "converts" _{...} into \low{...} and ^{...} into \high{...} (or possibly enable using both at the same time). \newbox\chemlowbox \def\chemlow#1% {\setbox\chemlowbox\hbox{{\switchtobodyfont[small]#1}}} \def\chemhigh#1% {\ifvoid\chemlowbox \high{{\switchtobodyfont[small]#1}}% \else \/\lohi{\box\chemlowbox}{{\switchtobodyfont[small]#1}}\fi } \def\finishchem% {\ifvoid\chemlowbox \else \iffluor \fluorfalse \kern-.1em \fi\low{\box\chemlowbox}\fi} % for "kerning" after F \newif\iffluor \unexpanded\def\molecule% {\bgroup \catcode`\_=\active \uccode`\~=`\_ \uppercase{\let~\chemlow}% \catcode`\^=\active \uccode`\~=`\^ \uppercase{\let~\chemhigh}% \dostepwiserecurse {65}{90}{1} {\catcode \recurselevel = \active \uccode`\~=\recurselevel \uppercase{\edef~{\noexpand\finishchem \rawcharacter{\recurselevel}}}}% \uccode `\~=`\F \uppercase{\def~{\finishchem F\fluortrue}}% \catcode`\-=\active \uccode`\~=`\- \uppercase{\def~{--}}% %\loggingall \domolecule }% \def\domolecule#1% {\expandafter\scantokens\expandafter {\detokenize{#1\finishchem}}\egroup} \starttext \molecule{CO_2} \stoptext Thanks for any hints, Mojca