Au(III)−Schiff base complexes are active hydrogenation catalysts, giving turnover frequencies similar to those of the corresponding complexes of Pd(II), which has the same d⁸ electronic structure as Au(III). The mechanism of the reaction has been studied in detail by a combination of kinetic experiments and theoretical calculations. It is predicted and tested that the nature of the solvent plays a critical role for the heterolytic cleavage of H₂ (controlling step). Taking this into account, and by properly selecting the nature of solid supports (polarity and proton-donating ability), it was possible to strongly increase the activity of the homogeneous Au(III) and Pd(II) catalysts by grafting them onto the surface.