The adsorption of hen’s egg lysozyme (LSZ) and milk alpha-lactalbumin (LAC) on colloidal AgI is described. These two proteins have similar shapes and sizes, but different structural stabilities and isoelectric points. The charge at the silver iodide surface is determined by the concentration of I- (or, for that matter, Ag+), whereas that at the protein surface is controlled by the pH. The influence of electrostatics on adsorption is studied by varying the pi, pH, and ionic strength. Desorption experiments show that LAC adsorbs more irreversibly than LSZ. Analysis of calorimetric data suggests that the adsorption of LAC does not depend on the surface coverage, whereas the adsorption of LSZ does. Proton titrations reveal, for both proteins, a strong influence of the adsorption on the titration behavior of the carboxyl groups. An estimation of the coadsorption of ions is presented by comparing proton titration and electrophoretic mobility data.