The stability of n-CdS in a Fe(CN64-/3- electrolyte is demonstrated to be due to the formation of a CdFe(CN62-/1-overlayer. Mediated hole transfer through the overlayer to solution Fe(CN64- competes with photoanodic decomposition of the electrode. The charge transfer energetics and kinetics of the overlayer are very sensitive to the supporting electrolyte cation. Variations in supporting electrolyte allow maximization of overlap between the semiconductor states and the filled levels of the surface CdFe(CN62/1 couple. This leads to enhanced energy conversion efficiencies and minimization of electrode decomposition. For the n-CdS electrode, a supporting electrolyte containing both K+ and Cs+ is found to maximize cell performance, A surface state responsible for deleterious electron transfer through the barrier is observed —600 mV positive of the conduction bandedge. Cells with monochromatic efficiencies in excess of 20% (488 nm) can be obtained by considering the CdFe(CN62-/1- overlayer properties.