The kinetics of the protein apotransferrin adsorbing onto Si(Ti)O-2 surfaces from solution is accurately measured using an integrated optics technique. Specifically, the influence of salt and buffer concentrations on the adsorption rate is investigated. Analysis of the kinetic curves allows the separate evaluation of the diffusivity and the area occupied per adsorbed molecule. As the ionic strength of the solution increases, the lateral diffusivity decreases and the effective area per molecule increases. These observations are analyzed in terms of the surface charge model of Healy and White, It is shown that increasing ionic strength diminishes the negative surface potential but increases surface pH and hence net protein charge near the surface. Including these parameters in the adsorption kinetic equations leads to good quantitative agreement with the experimental data over an intermediate range of ionic strengths, but not at the highest values investigated, where nonelectrostatic factors begin to play a role.