Several deep-level defects in hydrogen-passivated GaAs doped with Si have been investigated. The defect transformation by atomic hydrogen confirms the assignment of these defects to metastable defects associated with a hydrogen atom. Thermal-annealing experiments under biased and unbiased conditions confirm that during hydrogenation a deep level at 0.60 eV below the conduction band is generated, as a metastable defect for the native deep level at 0.42 eV below the conduction band, and the complete passivation of the 0.42- or the 0.33-eV trap during hydrogenation is due to passivation of the trap by a hydrogen-atom-forming hydrogen-defect complex. The first-order kinetics permits a precise estimate of the formation and annealing frequencies f and a of the hydrogen-defect pair. The temperature-dependent values of a for the 0.60-eV trap satisfy the relation a=(0.82×1013)exp[(-1.61±0.04 eV)/kT] s-1. We propose that this activation energy could be the value required for the release of a hydrogen atom bound to a point defect in GaAs.