Purple membrane fragments with its integrated protein bacteriorhodopsin were immobilized on lipid impregnated and lipid free cellulose acetate membranes. Two mathematical models were suggested to simulate the photoresponse curves of these membranes which were obtained by measuring the pH variation independently in each chamber of a specifically designed photoactivity cell for long periods of illumination. The first order model suggested was mainly based on the existence of light phase deprotonation and dark phase reprotonation reactions. The overall deprotanation rate constants were found to be slightly less than the overall reprotonation rate constants with the first order kinetics. In the second model, the pH change-time data were expressed by a mathematical decay function. There has been found a great agreement between the first order rate constants and the decay constants.