A process for preparing monocarboxylic esters of the general formula I ##STR1## where R.sup.1 and R.sup.2 are each hydrogen, C.sub.1 -C.sub.12 -alkyl, C.sub.3 -C.sub.8 -cyclo-alkyl acyl, aryl or C.sub.7 -C.sub.20 -aralkyl or together --(CH.sub.2).sub.n --X--(CH.sub.2).sub.m --,X is methylene, oxygen, sulfur, NH or NR.sup.3,R.sup.3 is C.sub.1 -C.sub.12 -alkyl, andn and m are each from 0 to 8, comprises reacting geminal dicarboxylic esters of the general formula II ##STR2## where R.sup.1 to R.sup.3 are each as defined above, at from 150.degree. to 400.degree. C. in the presence of catalysts.