To gain more insight into the process of formation of adsorbed layers of diblock copolymers, ellipsometry was used to monitor in situ the thickness and adsorbed amount of a series of polystyrene-b-poly(2-vinylpyridine) diblock copolymers adsorbed on silicon from toluene. First, it is demonstrated that ellipsometry can be sensitive to both the zeroth (Gamma(0)) and first (Gamma(1)) moments of the interfacial density profiles of the PS/PVP layers, providing sufficient information to measure the thickness 2Gamma(1)/Gamma(0) at each stage of the adsorption process. Second, it is found that in the early stages of layer formation, at the point where adsorbed chains are forced to laterally overlap, the thickness of the PS/PVP layers is equal to approximately 2.2R(g). As the adsorbed amount surpasses approximately twice this surface density, the thickness grows as the layer becomes progressively more crowded. In this growth regime, the dependence of the thickness on the adsorbed amount makes a transition to a one-third power-law at the highest adsorbed amounts.