Scanning-angle reflectometry :around the Brewster angle has been used to investigate the optical properties of positively charged polystyrene particles with radii ranging from 50 to 500 nm, deposited on a silica/water interface. A theory for inhomogeneous thin films may be used to analyze the data for the smallest of these particles, but gives incorrect results for the larger ones. A model based on Mie theory describes the data well over the entire size range. This model ignores direct particle substrate interactions, which is justified for the small differences in refractive indices in this system. It is shown that the Mie-based model is equivalent to the thin film model in the long-wavelength limit. The analysis yields three parameters : particle size, refractive index, and coverage.