The first controlled differential etching for profiling magnesium‐implanted semi‐insulating gallium phosphide is reported. Profiles of surface resistivity, mobility, and carrier concentration versus depth below the substrate surface for 129‐keV magnesium‐ion implants are obtained. Agreement was obtained with glow‐discharge optical spectroscopy data from an unannealed implanted substrate. The non‐Gaussian profiles indicate ion diffusion during annealing and possibly during implantation. Optimum procedures and techniques are developed for the substrate capping material, the anneal temperature, the capping method, and the application of Ohmic contacts. The carrier concentration of electrically active implanted magnesium ions in gallium phosphide, for a total magnesium‐ion dose of 1.0×1013/cm2, is the highest percent efficiency reported (∼44%).