Optical reflectometry was used to study adsorption behavior of poly(l-lysine) (PLL) on the water–silica interface for different pH and concentrations of the monovalent salt NaBr. At low salt level and low pH, the adsorbed amount increases with increasing salt concentration and pH. This adsorption behavior can be qualitatively explained by electrostatic interactions between the negatively charged substrate and the positively charged polyelectrolyte. At higher salt levels and pH, the adsorbed amount passes through a maximum. This maximum is probably related to the competition between adsorbing monovalent cations and the polyelectrolyte. The initial adsorption kinetics is mainly controlled by the mass transport to the surface.