Disclosed herein is a process for the preparation of optically active compounds of the formula: ##STR1## in which R is a lower alkyl, lower alkoxy, benzyloxy, phenyl, o-nitrophenyl, p-nitrophenyl, or p-tolyl group, or salts or esters thereof, which process comprises the following steps:(a) starting with a racemic mixture of the compound of Formula I in which R is phenyl, namely, (+)-3-benzoylthio-2-methylpropanoic acid;(b) adding thereto 0.40-0.60 moles of (+)-dehydroabietylamine per mole of (+)-3-benzoylthio-2-methyl-propanoic acid as the resolving agent, in the presence of an aliphatic carboxylic acid lower alkyl ester as the resolution solvent;(c) crystallizing out and isolating the (-)-3-benzoylthio-2-methylpropanoic acid, (+)-dehydroabietylamine salt, which is dextrorotary;(d) if necessary, recrystallizing said dextrorotary salt to further purify it;(e) hydrolyzing the resulting dextrorotary salt with a base to separate the (+)-dehydroabietylamine resolving agent from the (-)-3-benzoylthio-2-methyl propanoate;(f) hydrolyzing the (-)-3-benzoylthio-2-methyl-propanoate with an acid to form the acid thereof;(g) optionally, converting the phenyl group of the resulting (-)-3-benzoylthio-2-methyl-propanoic acid to one of the other R groups of Formula I.