Isotherms for the adsorption of poly(vinylpyrrolidone) (PVP) and bovine serum albumin (BSA) onto silica have been determined. The adsorption capacity for PVP increased with increasing molecular mass but was independent of pH and ionic strength. The adsorbed layer thickness, delta, was determined from zeta potential measurements. delta was found to increase with increasing surface coverage but was significantly less than the polymer radius of gyration even at the adsorption plateau, indicating that the polymer coils flattened at the surface. BSA did not adsorb onto silica from water at pH 7 because of electrostatic repulsions, but adsorption did occur in the presence of electrolyte. The amount of BSA adsorbed at plateau coverage increased from similar to0.1 mg/m(2) in 0.001 M NaCl to similar to1.0 mg/m(2) in 0.5 M NaCl. BSA adsorption was significantly inhibited but not completely prevented, by precoating the silica particles with PVP. Even at low PVP surface coverages, where delta had very low values, some inhibition occurred. Furthermore, PVP was able to desorb most but not all BSA molecules from the silica particles.