Neutron reflectivity (NR) was used to study the adsorption of human serum albumin and human fibrinogen on quartz. The proteins were individually and sequentially adsorbed from heavy water and heavy water/methanol mixtures at pH 4 and 7.0. The technique allows for the subnanometer resolution of the adsorbed layer thickness and gross morphology. Under the conditions of our measurements we found that fibrinogen formed a distinct layer that we interpret as a mat of the protein three layers thick whereas albumin formed only diffuse layers. The adsorption pattern of the two proteins changed radically when one protein was adsorbed oil top of the other (previously adsorbed). In general our Measurements indicate that the adsorbed protein layers on quartz are rather loosely bound and that these layers, incorporating as much as 80% water, extend further into the bulk fluid than might have been expected. (c) 2004 Elsevier Inc. All rights reserved.