The adsorption of some model proteins, human serum albumin (HSA), IgG, and fibrinogen, at silica made hydrophobic by either methylation or plasma deposition of HMDSO (hexamethyldisiloxane) was investigated with in situ ellipsometry and TIRF (total internal reflection fluorescence spectroscopy). Ellipsometry experiments of the simultaneous adsorption of HSA and either IgG or fibrinogen revealed a preferential adsorption of the latter proteins. This is seen not only from the total adsorbed amount, but also from the adsorbed layer thickness and from the buildup of the adsorbed layer. However, in sequential adsorption experiments, where HSA was first allowed to adsorb, followed by rinsing and addition of either IgG or fibrinogen, the additional adsorption is quite limited. Consequently, when HSA is allowed to adsorb on its own at hydrophobic surfaces, it is not removed by either IgG or fibrinogen to any larger extent. Furthermore, preadsorbed HSA was not exchanged by HSA added after adsorption and rinsing, implying that the mechanism behind this effect is an "irreversible" adsorption of HSA at hydrophobic surfaces, possibly originating from a surface-induced conformational change of HSA at these surfaces. Analogous findings were obtained with both methylated and HMDSO-treated surfaces, using both ellipsometry and TIRF.