The adsorption of some model proteins, human serum albumin (HSA), IgG, fibrinogen, and lysozyme, at methylated silica surfaces was investigated with in situ ellipsometry. By performing studies with the bare substrate at two different ambient refractive indices and using four-zone averaging, the adsorbed amount, the adsorbed layer thickness, and the mean adsorbed layer refractive index are obtained. The adsorbed amounts obtained for the proteins agree well with previous results. The adsorbed layer thicknesses vary strongly between the proteins. Thus, at adsorption plateau, the adsorbed layer thicknesses (delta(el)) obtained for HSA, lysozyme, IgG, and fibrinogen are 4 +/- 2, 11 +/- 2, 18 +/- 2, and 28 +/- 2 nm, respectively. The buildup of the adsorbed layers proceeds differently for different proteins. Thus, for fibrinogen, both delta(el) and the adsorbed layer mean refractive index (n(f)) increase monotonically up to about 4 mg/m2. For IgG, on the other hand, delta(el) is essentially independent of the adsorbed amount, whereas n(f) increases linearly. Finally, the adsorbed layer formed by lysozyme is more compact than those formed by fibrinogen, IgG, and HSA. These findings are discussed in terms of adsorbed layer structure.