The passivation of a bulk donor defect (Ec–0.36 eV) in polycrystalline GaAs by reaction with atomic hydrogen has been observed using deep level transient spectroscopy. The concentration profile of the defect is presented as a function of the duration and temperature of the exposure to atomic hydrogen—when exposed for 3 h at 250 °C, 3×1015donor defects cm−3were passivated to a depth of ∼1.1 &mgr;m.