Homogeneous and heterogeneous catalysts with water-tolerant Lewis acids, such as Sc(OTf)(3) and Nb2O5 center dot nH(2)O, effectively promote the hydride transfer of pyruvic aldehyde into lactic acid in water at 383 K, whereas Bronsted acid catalysts cannot function as effective catalysts under mild conditions. NMR measurement in the presence of D2O suggests that the reaction on a Lewis acid catalyst proceeds via the Meerwein-Ponndorf-Verley (MPV) reduction.