By combining photo-electrochemical and photo-induced microwave conductivity measurements, information on potential dependent minority charge carrier accumulation, on interfacial minority carrier concentration and on interfacial charge transfer rates can be obtained. It suggests a correlation between electron transfer processes and accumulated charge carriers dominated by non-equilibrium conditions. This is inconsistent with the general assumptions leading to the classical Marcus-Gerischer electron transfer at electrodes, conceived for weak interaction, quasi-equilibrium and absence of polarisability effects. It is considered only to be applicable in special situations. A non-linear interfacial electron transfer theory, the properties of which are outlined, will on the other hand open the potential for new phenomena. They include faster (stimulated), and cooperative electron transfer. The latter, which is excluded by the classical theory, requires non linear dynamic feedback polarisability, which will have to be developed on the basis of structural-electronic considerations for semiconductor interfaces to become highly catalytic. (C) 2003 Elsevier Ltd. All rights reserved.