The mechanism of the photoelectrochemical (PEG) etching of p-InP in nitric acid was investigated. The success of the procedure is shown to be due to the chemical oxidation of the photoproduced metallic indium by nitric acid. The study gives insight into how the photoreduction of p-InP and protons occur simultaneously Plots of the etch rate vs. nitric acid concentration for two p-InP electrodes with different carrier concentrations and etch pit densities (EPD) indicated that defects play a role in the photoreduction process. The effect of EPD on the photoreductions;has been interpreted in terms of surface states which originate from EPD; the EPD are correlated with the magnitude of the surface states. The total number of coulombs passed in the PEC etching region was found to be relatively constant for each p-InP electrode. Comparison of the charge transferred with the PEC etch rates indicated that the majority of protons are photoreduced via surface states. A positive shift of the onset potential for photocurrent with increasing nitric acid concentrations implied a decrease in the density of surface states. This observation was correlated with the participation of the conduction band in the photoreduction of p-InP. At higher nitric acid concentrations, experiments indicate that the photoreduction of protons is possible at both the surface states and conduction band. Energy diagrams lend support to these suppositions.