Neutron reflection was used to characterize the adsorbed layer structure for lysozyme conjugated at the N-terminus with a single perdeuterated methoxy poly(ethylene glycol) (PEG) chain at the silica/water interface. Adsorbed layers were produced with two different surface concentrations corresponding to opposite sides of a pronounced transition in the adsorption isotherm for mono-PEGylated lysozyme. The transition was previously ascribed, on the basis of less direct characterization by normal force measurements, to a change in the distribution of the conjugated PEG chain segments in the interfacial region in response to lateral repulsions (S.M. Daly, T.M. Przybycien, R.D. Tilton, Langmuir 21 (2005) 1328-1337). Neutron reflectivity was measured for both surface concentrations using three different sets of neutron scattering length density contrast conditions, and models for the distribution of PEG and lysozyme content in the layers were obtained by simultaneous regression of all contrast condition data sets. This analysis indicated that the surface proximal volume is occupied almost equally by PEG and lysozyme at the low surface concentration, while at the higher surface concentration PEG is preferentially shifted away from the surface, with all lysozyme remaining in the surface proximal region. The distal regions of the high surface concentration layer contain only PEG, consistent with the previous interpretation of normal force measurements. (C) 2012 Elsevier Inc. All rights reserved.