Electrochemical quartz crystal microbalance (EQCM) measurements provide compelling evidence for charge-compensating cation uptake by nanocrystalline SnO2 and ZnO electrodes during electron addition. Comparative light water/heavy water measurements establish that the adsorbed or intercalated ions are protons or deuterons. Additional studies as a function of pH implicate water, rather than hydronium ions, as the proton source. The new results, when combined with previous results for titanium dioxide in nonaqueous electrolytes, suggest that charge-compensating cation intercalation is a general mode of reactivity for metal oxide semiconductors. Finally, the new observations raise significant fundamental questions concerning (1) chemical control of band energetics, (2) possible band-edge-unpinning phenomena, and (3) relationships between band edge energies and driving forces for isolated electron transfer reactions.