Porous silicon layers with thicknesses in the range 100-400 nm and average porosities in the range 38-71% were prepared by electrochemical anodization. Variable angle spectroscopic ellipsometry was used to characterize the microstructure of the layers before protein adsorption. In-situ ellipsometry was then employed tr, monitor the kinetics of fibrinogen and human serum albumin adsorption. At steady state new ellipsometric spectra were recorded to determine the total amount of adsorbed protein. Under the experimental conditions used here, the protein molecules were found to adsorb in the outermost part of the porous layer. However, human serum albumin penetrated into the porous silicon matrix at low pH and high porosity. From a methodological point of view it was found that spectroscopic ellipsometry is an appropriate tool for characterization of the microstructure of porous silicon layers and for in-situ monitoring of protein adsorption in such layers including depth profiling.