Oxygen has been introduced by ion implantation into zinc-doped epitaxial gallium phosphide grown from the vapour phase. After annealing, cathodoluminescence from zinc-oxygen pairs is observed, having an efficiency approaching that of bulk zinc-oxygen doped solution-grown material. For a zinc concentration of 2 × 1017cm−3, an optimum oxygen concentration for cathodoluminescence of 3 × 1018cm−3 is exhibited.