Heterogeneous two-dimensional layers have been the subjects of many ellipsometric studies. An unambiguous characterization of especially metal heterogeneous layers is not straightforward. Adsorption of Au nanocolloids leads to a well-defined heterogeneous layer, although the analysis of ellipsometric spectra is not simple. Standard effective medium theories are shown not to be capable of a correct description. The thin island film theory developed by Bedeaux and Vlieger (Optical properties of surface Imperial College Press (2002)), which evaluates a spherical An colloid as an excess polarizability at the surface, provides a good framework for the analysis. Both the image dipole and the lateral distribution can be incorporated in a straightforward manner. The influence of the change in particle shape from spherical to prolate is evaluated and shows that already a small change in shape leads to large influences in the optical response. This strong effect is due to the influence of shape on the depolarization factors and especially due to the large optical contrast between the Au colloids and the ambient. In contrast to this, the adsorption of the strongly aspherical protein fibrinogen can be analyzed straightforwardly. Only the adsorbed amount of protein on the surface is recorded in-situ. This is the result of the minimal refractive index contrast between proteins and water, which results in a negligible influence of shape on the optical response. (C) 2004 Elsevier B.V. All rights reserved.