The coadsorption of bovine serum albumin (BSA) and trace quantities of its methylated derivative obtained by reductive alkylation ((CH3)-C-14-BSA) on montmorillonite has been studied by a batch method involving a first adsorption stage of BSA for a time tau1, followed by a second adsorption stage of CH3-BSA for a time tau2. The periods tau1 and tau2, the concentrations of BSA and CH3-BSA, and the pH have been varied. The influence of tau2 on the CH3-BSA binding has shown the existence of a barrier to the adsorption or exchange on the surface both above and below the protein isoelectric point (IEP). From the effects of the BSA concentration in the first adsorption step and of its aging time tau1, it was deduced that above the IEP the repulsive interactions were electrostatic (bare electronegative mineral surface) and that below the IEP they were steric (monolayer of preadsorbed BSA on the surface). The higher affinity of the methylated protein for the clay surface was discussed in relation to the origin of the hydrophilicity of montmorillonite (hydrophilic exchangeable charge-compensating cations, but hydrophobic siloxane layer) as was the effect of conformational changes on the exchangeability of a preadsorbed protein layer.