Transitions of the deep O donor in a GaPp‐njunction are studied by photocapacitance. It is found that the O donor may deeply trap either one or two electrons. The four optical absorption cross sections are measured for the two states, and from this information the O concentration profile is determined. The concentration ranges from (4 ± 1) × 1014cm−3on thenside to (1.5 ± 0.2) × 1016cm−3on thepside of the junction depletion layer. It is believed that these measurements represent the first direct determination of the concentration of O donors in GaP.