We have used specular neutron reflection, employing contrast variation by isotopic substitution, to probe the structure of a poly(ethylene glycol) (PEG) coated silicon surface. A PEG molecule with a molar mass of M w = 5000 g mol - 1 and a trimethoxysilane end group was chemisorbed onto the silica surface. The grafted layer was found to possess an average thickness of ∼85 A in its solvated state and the surface coverage was Γ P E G = 1.1 ′ 0.2 mg m - 2 , corresponding to an area per molecule of A P E G = 770 ′ 230 A 2 . For this surface coverage, brush formation is indicated since the layer thickness is found to be approximately three times the radius of gyration, R g , of the PEG molecule, and the average area per molecule is consistent with overlapping polymer chains. We have further investigated the structure of the PEG layer when bovine serum albumin (BSA) is present in the aqueous solution. The protein adsorption measurements were conducted at pH = 5, close to the isoelectric point of BSA, where its maximum adsorption is found when adsorbed on to silica, and the concentration of the protein solution was 0.1 g dm - 3 . When BSA is present in solution the thickness of the PEG layer is reduced to ∼55 A, but, within the sensitivity of the present measurements, there is no evidence for the formation of an adsorbed BSA layer. Control experiments were performed against a hydrophilic silica surface.