Solvent isotope effect studies were undertaken with the NAD-malic enzyme from Ascaris suum. With different varied substrates and divalent metal ion activators, nearly all the measured isotope effects are inverse, with the effect on V/K more inverse than the effect on V. The usual explanations for inverse solvent isotope effects, such as medium effects, involvement of a thiol group, or dissociation constant of a metal-chelated water, were unsatisfactory in the NAD-malic enzyme reaction. The observation of a close correspondence between changes in kinetic parameters induced by increased solvent viscosity with glycerol as viscosogen and the inverse solvent isotope effects has led to the conclusion that solvent effects are the result of the difference in viscosity between D2O and H2O.