A change in the dinucleotide reactant from NAD⁺ to the more oxidizing APAD⁺ in the malic enzyme reaction results in a change in the mechanism of oxidative decarboxylation of malate from stepwise to concerted. In order to determine whether this is a phenomenon general to metal ion dependent b-hydroxyacid oxidative decarboxylases, tartrate dehydrogenase, which catalyzes a reaction diastereotopic to malic enzyme, was studied using the technique of multiple isotope effects. A primary deuterium isotope effect of 1.41 on V/K_malate was measured, as well as a primary ¹³C-isotope effect of 1.0096. A decrease in the measured value of the ¹³C-isotope effect to 1.0078 is consistent with a stepwise mechanism, as observed for malic enzyme. The ¹³C-isotope effect with thioNAD⁺ also decreases from a value of 1.0053 using D-malate to 1.0009 using D-malate-2-D, consistent with stepwise oxidative decarboxylation with this alternative dinucleotide substrate. The data suggest that the change from a stepwise to a concerted mechanism with malic enzyme may be a unique phenomenon.