Two different asymmetric hydrogenation methodologies based on gaseous hydrogen at 1 atm pressure and transfer hydrogenation from the decomposition of formic acid, in the presence of a rhodium catalyst, have been used to prepare (S)-(-)-methylsuccinic acid enriched with the deuterium isotope. NMR and mass spectroscopic evidence indicates that complex labeling occurs. We interpret this labeling pattern as the result of an equilibrium which exists between the olefin and a catalyst-alkyl intermediate in a Wilkinson-type mechanism. This phenomenon has not been reported in similar experiments with Q-phenylitaconic acid where the expected cis-addition of deuterium from either deuterium gas at 1 atm, or deuteriated formic acid under transfer hydrogenation conditions, was observed. In our studies reducing itaconic acid (methylenebutanedioic acid), there was no loss of asymmetric induction (above 90% ee), approximately 2.4 deuterons were incorporated into (S)-(-)-methylsuccinic acid, a ratio of 1.8:1 methyl : methine deuteriation was observed, and there was no evidence for olefin isomerization into conjugation with both carboxylic acid groups. Following these isotopic enrichment studies, we present the first evidence for reversible transfer addition of deuterium to methylsuccinic acid, either by gaseous or transfer hydrogenation at atmospheric pressure. Such a mechanism has recently been eliminated for reduction at elevated pressures. These results have general applicability to the synthesis of isotopically labeled homochiral substituted carboxylic acids and also in interpreting the C-13 NMR data which are generated by the simultaneous presence of several deuterium containing isotopomers.