The dissolution and recombination processes at the CdS/electrolyte interface have been studied with the electrical and opto-electrical impedance methods. It has been found that dissolution intermediates can act as recombination centers and that thermal excitation of electrons from these intermediates to the conduction band can occur. In the case of the opto-electrical impedance it is shown that the modulation of the light intensity results in a modulation, not only of the minority carrier concentration, but also of the majority carrier concentration. As a consequence, a value for the Helmholtz capacitance could be determined from the opto-electrical impedance results