Experimental data of dependence of photopotential of germanium and compounds A^IIIB^V of gallium and indium on properties of semiconductor (type and concentration of the doping impurity, width of a prohibited zone), composition of electrolyte (pH, concentration of oxidizers and restorers), kind of illumination are analyzed. It is shown that in darkness empirical values of electrode potentials of semiconductors are close to thermodynamic ones, calculated by energies of Gibbs for reactions with participation of oxides of lowest degrees of oxidation: GeO, Ga₂O, In(OH)₂ and B(0); for them value of standard potentials is maximum. At illumination potentials of undegenereted semiconductors of n-type come to theoretically calculated values for reactions with participation of their oxides in a higher degree of oxidation: GeO₂, Ga₂O₃, In₂O₃ and В₂О₃. For them standard potential is appreciably lower.