The photo-electrochemical behaviour of glassy carbon was investigated in the range of low anodic potentials, both in a supporting electrolyte alone and in the presence of electron donors (Fe2+,Ru(NH3)62+). The amount of the observed photo-current and the shape of the spectral photo-response are shown to be strongly affected by the nature of the effective electron donor present in the solution. The largest photo-currents, exceeding by far the “dark” currents were observed for the oxidation of Fe2+ ions, known to be catalysed by the CO groups on the surface of glassy carbon. Only marginal photo-currents, associated with the oxidation of functional groups on the surface of glassy carbon, were recorded in the supporting electrolyte.