Abstract
Semiconducting 
evolves oxygen under anodic polarization despite the presence of an exhaustion layer. Addition to the solution of species more oxidizable than water causes no increase in current at a given potential, hence electron emission over the surface barrier is unimportant. The data may be interpreted in terms of electron tunneling from oxide ions in the crystal surface as the rate‐determining step followed by faster electrochemical processes. Implications for electrolytic rectification are discussed.