Hydrogenated single crystal (100) diamond surfaces subjected to an electrochemical (EC) treatment are selectively oxidized at room temperature. Part of the surface remains hydrogenated, except for a narrow transition region between the oxidized and hydrogenated regions. Ultraviolet photoelectron spectroscopy indicates that the transition region has negative electron affinity (NEA), as do the surfaces of hydrogenated crystals. The oxidized and hydrogenated parts of the EC‐treated surfaces do not have NEA. A possible explanation is that contaminants eliminate NEA in the hydrogenated parts of the EC treated surfaces, but the transition region remains uncontaminated. None of the oxidized surfaces exhibit NEA. ©1995 American Institute of Physics.