Single crystalline n-type GaP was made porous by anodic etching in the dark in sulfuric acid solution. The morphology and (photo)electrochemical properties of the porous layer were investigated. Submicron pits 10 to 30 μm apart were formed at the surface. Below the surface, the pits acted as the nuclei of distinct porous domains. Pore widths and pore wall thicknesses were approximately 100 nm. The creation of the porous layer caused the quantum yield for light-to-current conversion to increase from extremely low values to unity for light absorbed in the indirect optical transition. The mechanism of pore formation and the striking opto-electrical properties of porous GaP are discussed.