The adsorption of some model proteins, human serum albumin (HSA), IgG, fibrinogen and lysozyme at silica surfaces was investigated with in situ ellipsometry and compared to previous results obtained for methylated silica surfaces. The adsorbed amount (G), the adsorbed layer thickness (del) and the mean adsorbed layer refractive index (nf) were obtained by a procedure involving studies of the bare substrate at two different ambient refractive indices, as well as four-zone averaging. The surface hydrophobicity strongly influenced the adsorption properties of all the proteins studied. For HSA, IgG and fibrinogen, the adsorbed amount was significantly lower at the hydrophilic surface than at the hydrophobic one, whereas the reverse was found for lysozyme. For fibrinogen, the adsorbed layer thickness at silica was smaller than that at methylated silica, whereas the adsorbed layer was more concentrated. For IgG, on the other hand, end-on adsorption was observed at both silica and methylated silica. For lysozyme, side-on adsorption in a dense monolayer was observed at silica, whereas at methylated silica, the adsorption occurs in 2-3 rather dilute molecular layers. Furthermore, the build-up of the adsorbed layers was studied. For fibrinogen, qualitatively the same behaviour was observed for silica and methylated silica, i e, as the adsorbed amount increases, both del and nf increase initially, while closer to adsorption saturation, nf levels off. However, at a given G, del was lower and nf higher at silica than at methylated silica. A similar finding was obtained for lysozyme. These findings are discussed in terms of the adsorbed layer structure and formation.