The adsorption of bovine serum albumin (BSA) at the hydrophilic silica-water interface has been studied using specular neutron reflection. The measurements were made over the concentration range from 0.005 to 0.5 g dm(-3). The surface excess was found to vary from 1.8 to 2.4 mg m(-2). The layers could be modeled using a single uniform layer model, suggesting that over the concentration range studied then is insignificant denaturation, which would lead to a more fragmented peptide distribution and hence layers of different density. Comparison of the layer thickness with the dimensions of the ellipsoidal structure of the globular solution structure indicates that the molecules adsorb sideways-on. Nevertheless, the layer thickness is always less than 40 Angstrom, suggesting that adsorption onto the hydrophilic surface results in some structural deformation. The increase of layer thickness with bulk concentration suggests that the extent of the distortion is reduced as the lateral repulsion between protein molecules increases. The effect of pH on the adsorbed BSA layer was examined by varying the pH at a fixed BSA concentration of 0.15 g dm(-3). The cycle was started at a pH of 5.1, followed by pH 7, 5.1, 3, and then back to 5.1. The neutron reflectivity profiles showed no hysteresis in either adsorbed amount or structure. The reversibility of the adsorption of BSA with respect to pH is consistent with no denaturation occurring on the surface. The adsorbed amount was a maximum at pH 5.1, which is close to the isoelectric point at pH 4.8.