The exchange of majority carriers between a semiconductor electrode and a simple redox system is discussed. It is assumed that tunneling through the Helmholtz-layer occurs iso-energetically, The electronic states of the redox system are described by the fluctuating energy level. model. The dependence of the potential drop over the Helmholtz-layer on the rate of tunneling is quantitatively taken into account. Direct exchange between the majority carrier band and the redox system has been compared with surface state mediated exchange. It is shown that, in the case of surface state mediated transfer, the potential distribution over the depletion-and Helmholtz-layer is coupled to the interfacial kinetics. The current-potential characteristics pertaining to surface state mediated electron transfer may largely deviate from those pertaining to direct exchange. Experimental results from the literature are reviewed in the framework of the model.