Under continuous illumination, isolated planar sheets of purple membrane from Halobacterium halobium acidify the surroundings at alkaline pH. This light-induced change in H+ binding to the purple membrane (delta h) was studied by differential titration under varying conditions of pH, temperature, ionic strength, salt composition, light intensity, and wavelength. A maximum acidification was found between pH 9 and 10, with delta h less at neutral or more alkaline pH, consistent with a previously proposed three-state model. The light intensity and wavelength dependence also support this model. The temperature dependence of delta h, interpreted in terms of the three-state model, is anomalous. The apparent enthalpy of proton dissociation (delta H0) is -6 kcal/mol, a value of opposite sign to the expected delta H0 for a group of pK = 10. The apparent activation energy (Ea) for proton uptake is 14 kcal/mol in 15 mM NaCl and 18 kcal/mol in 3 M KCl, 5 to 10 times too large for a diffusion-limited proton transfer reaction from water. However, both delta H0 and Ea are consistent with conformational changes linked to light-independent proton dissociation and pump-dependent proton uptake. An increase in ionic strength increases delta h. This effect is shown to be quantitatively explained by a high negative electrostatic surface potential, which accumulates protons in a diffuse electrical double layer.