The adsorption of lysozyme onto silicon oxide surfaces was investigated using in situ ellipsometry. Both the adsorption kinetics and the desorption upon rinsing with buffer were followed and the experimental data obtained were compared to models for adsorption kinetics. The effects of concentration, time of adsorption, and ionic strength were studied. The adsorbed amount reached a semiplateau within an hour of adsorption and only partial desorption was observed upon rinsing. The adsorption was found to be affected by the ionic strength and the following parameters were seen to increase in buffers of low ionic strength : the amount adsorbed, the rate of initial adsorption, and the amount adsorbed irreversibly after rinsing with buffer. The amount adsorbed after 1 h did not reach a plateau within the concentration range investigated, in spite of reaching levels above monolayer coverage. The fraction adsorbed irreversibly increased during the time interval studied(1 h). The experimental data could be fitted to a model involving lysozyme adsorbed in three states. These include molecules adsorbed reversibly to the surface, models adsorbed irreversibly to the surface, and molecules adsorbed in a second layer. The second layer was considered to adsorb irreversibly to the molecules in the first layer. Furthermore, the rate constants were dependent on the surface coverage and the model allowed for exchange between adsorbed molecules and those in solution.