We have performed positron lifetime experiments under monochromatic illumination in undoped semi-insulating GaAs. A negative vacancy, identified as the Ga vacancy, is observed in darkness. Illumination with 1.42 eV photons below 150 K reveals another type of vacancy, identified as the As vacancy. The As vacancy has a negative charge state above the ionization level at 50±5 meV below the conduction band. This level offers a microscopic explanation to the optical near-band-edge absorption. The concentrations of both Ga and As vacancies are between 10 15 and 10 16 cm -3