Dimethyl, diethyl and dipropyl esters of (Z)- and (E)-2-methyl-butenedioic acids (citraconic and mesaconic acid) were hydrolysed by pig live esterase, PLE. For the diethyl esters the enzyme showed low regioselectivity for both the (Z)- and the (E)-isomers. However, the hydrolysis of dimethyl and dipropyl (Z)-2-methyl-butenedioic acid gave a ratio of the monoesters of 95:5 and 85:15.