The photoelectric response of a windowless reflective silicon photodetector for obliquely incident polarized monochromatic light is used to characterize the native passivation dielectric thin film at the detector surface. In the presence of adsorption, the generated photoelectric signal becomes a function of the thickness and refractive index of the adsorbed layer. Higher sensitivity is achieved with s-polarized light and at incidence angles > 60 degrees. Furthermore, the passivation layer can be optimized for a specific adsorption experiment. A sensor of this kind is used to study the adsorption kinetics of H2O at the detector surface and the results are interpreted by Bruggeman’s effective medium theory. If a second detector is introduced to intercept the light reflected by the first detector, and the polarization of the incident light is modulated, analysis of output signals of this two-detector system determines the responsivity, absolute reflectances, and ellipsometric parameters of the first detector surface.