A theoretical model based on the hard-core two-Yukawa potential, and taking into account the surface diffusion and molecular diffusion, has been developed to predict the diffusion coefficient of enzymes on adsorption processes. The theoretical diffusion coefficients predicted by the model have been compared to the experimental diffusion coefficients of dilute aqueous solutions of catalase and Cu,Zn-superoxide dismutase (Cu,Zn-SOD) at different pH values, as determined by dynamic methods for immobilized metal-ion affinity chromatography (IMAC). For SOD, the experimental and the theoretical data show an increase in the relative diffusion coefficient as the enzyme net charge is increased. This behavior is due to the presence of unscreened charge in the enzyme surface. However, for catalase, the experimental the theoretical data show a decrease in the relative diffusion coefficient as the enzyme net charge is increased. This behavior is a consequence of a hydrodynamic expansion of the molecule produced by changes in the pH of the medium.