A differential response generated from a retinal protein, bacteriorhodopsin, at the electrode/electrolyte interface in an electrochemical cell was found to be a capacitive current caused by a potential change on the electrode that responds to a pH change arising from proton release and uptake during illumination of bacteriorhodopsin. The molecular mechanism for the proton-pumping of bacteriorhodopsin was rationalized by comparing capacitive currents of several mutant proteins that had been displaced the key amino acids for proton transfer. An extended study based on this method revealed that the other retinal proteins, halorhodopsin and phoborhodopsin, are also able to release and uptake protons under certain conditions.