In acidic solution, in the presence of SiMo12O404−, a heteropolyanion, n-InP is spontaneously photoetched. Indeed, under illumination (photon energy >1.35 eV) the polyanions are reduced while the semiconductor undergoes an anodic dissolution process due to the photogenerated holes. It has been established that the reduction of SiMo12O404− is achieved by electron capture from the InP conduction band. However, one question remained: is the electron capture direct from the InP conduction band or indirect via band gap surface states. In this work, evidence for a two-step mechanism is brought by electrochemical impedance spectroscopy. At the highest tested potentials, the frequency response is dominated by the space-charge capacitance, showing a perfect Mott−Schottky behavior and a flat-band potential of −1.03 V/MSE. For lower potentials, at which cathodic current is flowing, there is evidence for a 70 mV negative band edge shift and a corresponding additional capacitance peak assigned to charge relaxation effects in surface states exchanging electrons both with the InP conduction band and the SiMo12O404− species present in solution. These states, which are mediators for electron transfer, are located at 0.53 and 0.63 eV with respect to the conduction band minimum.