A new relation between the temperature change at a photoelectrode and the quantum efficiency of the photoelectrochemical reaction is derived from thermodynamical treatment in a photoelectrochemical process. Using this relation, the quantum efficiencies of the photoanodic reaction on titanium dioxide in aqueous solutions containing various hydroxy acid salts were evaluated. Addition of alpha-hydroxy acid salts caused a high increase in quantum efficiency, whereas addition of beta- and gamma-hydroxy acid salts caused a low increase.