The kinetics of electron transfer between electrochemically passivated chromium and several outer‐sphere redox couples have been investigated as a function of passive‐film thickness, to 25 Å, in . The apparent rate of electron transfer decreased exponentially with increasing thickness of the passivating overlayer. Similarly, during potentiostatic film growth, the electron‐transfer rate decreased logarithmically with time, , as a consequence of the film‐growth kinetics, since varies with . These relationships indicate an electron‐tunneling controlled process. However, variations in the film thickness and defects result in electron transfer occurring at distributed sites of enhanced tunneling probability, making a meaningful quantitative analysis difficult.