The p-GaAs photocathode/acetonitrile interface was characterised in the dark and with tens of mu W/cm(2) laser powers at 830 nm wavelength, using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Two redox mediators were used that present completely different behaviours under low optical excitation: one (Tris(2,2'-bipyridine)ruthenium(II) hexafluorophosphate) is sensitive to the surface states of the photocathode and the other (benzoquinone) is not. The combined electrochemical methods allow us to characterize the energetics of the electrode-electrolyte interface as well as the dominant electron transfer process.