Hydrogenations in aqueous systems with the soluble [Pd(alizarin monosulfonate)(2)] catalyst resulted in extensive deuteration of crotonic, trans-2-pentenoic and itaconic acids regardless of whether the deuterium source was D-2 or D2O. Itaconic acid was deuterated up to 3.6 D/methylsuccinic acid. Detailed H-1- and C-13-NMR studies identified six isotopomers of the deuterated methylsuccinic acid product and revealed an important role of the H/D exchange on the catalytically active Pd-intermediate.