Abstract A photoinduced admittance enhancement has been observed on n-GaAs and n-GaP electrodes in the potential range between flatband and stationary photocurrent onset. In order to provide a theoretical evaluation, the alternating current response of a semiconductor electrode under illumination has been investigated on the basis of non-equilibrium treatment of the carrier balance in the semiconductor and of the interfacial charge transfer kinetics. Superposition of an irreversible stationary and small-amplitude periodic rate has been treated for the following cases of charge transfer at the interface: (a) one-step electrochemical process; (b) two-step electrochemical process including an adsorbed intermediate and partial charge transfers; (c) parallel couple of one-step electrochemical and partial charge transfer chemisorption process. Empirical criteria for preference of charge transfer over surface recombination have been considered. In connection with the present development, the general equivalent circuit of a semiconductor electrode has been briefly derived from the dynamical charge balance. The theoretical approach of the stationary photocurrent-voltage curve has been discussed and refined.