The adsorption of β-casein at hydrophobic and hydrophilic silica surfaces has been studied by time-resolved ellipsometry. Marked differences in, e.g., adsorption kinetics and plateau adsorption coverage, were observed on the two types of surfaces. The miscellaneous adsorption mechanisms at the two surfaces resulted in different structures of the adsorbed layers as evident from the thicknesses and protein densities measured on the two substrates as well as the effect on the adsorbed layer properties of a subsequently added specific proteolytic enzyme, endoproteinase Asp-N. At the hydrophobic surface, the adsorption is fast and the surface is saturated within a relatively short period. The addition of endoproteinase Asp-N reduces the surface excess and the thickness by 24 and 45%, respectively. This corresponds to cleavage at amino acid residues 43 and/or 47 in the hydrophilic portion of the protein. Adsorption from solutions containing added electrolyte leads to significant increase of the surface excess. ...