The adsorption of chicken egg white lysozyme at the functionalized silicon oxide-solution interface has been studied using the combined measurement of spectroscopic ellipsometry and neutron reflection. The solid oxide surface was modified by coating a self-assembled monolayer of pentadecyltrichlorosilane with terminal hydroxyl groups (abbreviated to C15OH) Neutron reflection measurement at the solid-D2O interface showed that the C15OH layer was 16 +/- 2 PI thick and the volume fraction was 0.94 +/- 0.05, suggesting the formation of a close-packed monolayer. The adsorption of lysozyme was made at pH 4 and 7 with lysozyme concentration ranging from 0.03 to 4 g dm(-3). The results were then compared with those from previous studies at the hydrophilic SiO2-water and the hydrophobed SiO2-water interfaces, with the latter formed by coating a monolayer of octadecyl trichlorosilane (abbreviated to OTS). At 0.03 g dm(-3) and pH 7 the surface excess was found to be 0.6 +/- 0.3 mg m(-2) at the C15OH-water interface, as,compared with 1.7 mg m(-2) at the SiO2-water interface and 1.9 mg m(-2) at the OTS-water interface. As lysozyme concentration is increased to 4 g dm(-3), the surface excess at the C15OH-water interface reaches 2.1 mg m(-2), as compared with 4.7 mg m(-2) at the hydrophilic SiO2-water interface and 5.1 mg m(-2) at the OTS-water interface. These values demonstrate the attainment of the minimum surface excess on the hydroxyl surface. Shifting solution pH from 7 to 4 reduces adsorption on all the surfaces studied, but the lowest level of adsorption is again obtained on the hydroxyl surface. The reversibility of the adsorption at the C15OH-water interface was examined by cycling the solution pH at different lysozyme concentrations. Adsorption was found to be completely reversible at the low lysozyme concentration of 1 g dm(-3), while at the high concentration of 4 g dm(-3) the adsorption was irreversible.