Recent investigations of the photoelectrochemical (PEC) etching of p-InP in nitric acid showed an unusual property where hydrogen is not evolved with the formation of metallic indium. Zn order to fend the cause of this lack of hydrogen evolution at the irradiated p-InP electrode, the inhibition of the reduction of nitric acid was studied. Sulfamic acid has been shown to completely inhibit the reduction of nitric acid. With sulfamic acid, hydrogen is evolved at the irradiated p-InP electrode in nitric acid. The lack of hydrogen evolution in the PEC etching is attributed to the reaction of the nascent hydrogen atoms, formed on the p-InP electrode, with an active species involved in the reduction of nitric acid. In the presence of sulfamic acid, the hydrogen atoms combine to evolve hydrogen. Experiments showed that the reduction of nitric acid has no effect on the PEC etching of the p-InP in nitric acid.