Thin layers of ferritin adsorbed on gold substrates were studied in situ in buffer with spectroscopic ellipsometry. A four-phase model was used to determine their optical properties. The top layer was the protein layer and the bottom layer an effective medium layer used to model interaction between the protein molecules and the gold substrate. An analysis of the time dependence of the ellipsometric data showed that the adsorption can be described as a two-state process, The molecules adsorb first in a precursor state from which they can adsorb in a second state involving an electronic interaction with the gold substrate. The interaction with the gold was found to grow stronger with time. At the same time the layer thickness decreased and the layer density increased, The evaluation of ellipsometrically determined thicknesses and surface concentrations is also discussed.