The response of plasticized polyvinyl chloride membranes containing tetraphenylporphyrin on the pH of the aqueous solution was systematically studied. The concentration of teraphenylporphyrin in the membrane, the polarity of the membrane solvent, and the background electrolyte of the aqueous solution (NaCl, NaNO₃, and NaClO₄) were varied. A model was proposed, that allow computer simulation of experimental E-pH curves for porphyrin membranes. The model calculations were shown to fairly fit experimental data. Parameters responsible for the shape of the E-pH curves were estimated.