Protein monolayers were built up by allowing protein molecules in solution to deposit at the solid/liquid interface. The structural evolution of these layers was followed using optical waveguide lightmode spectroscopy (OWLS). The measured effective refractive indices were interpreted using the uniform thin film model. For a relatively rigid, spheroidal protein (transferrin), the thickness of the layer was already established at very low surface coverages, and the refractive index increased during deposition. For a highly elongated and flexible protein (fibronectin), the converse occurred. The farmer case is consistent with classical random sequential adsorption. The latter result implies constant conformational rearrangement during deposition producing a compact proteinaceous membrane.