We show that silicon acceptors can be neutralized in p-type GaAs: Si. SIMS analysis shows that the deuterium concentration closely follows the net acceptor concentration. Infrared absorption spectroscopy reveals lines at 2094.7cm-1 and 1514.5cm-1 in hydrogenated and deuterated samples respectively. The ratio r of the two frequencies is 1.383 indicating that the lines are related to hydrogen isotopes. The reduction of the intensity of the local vibrational mode (LVM) of SiAs after neutralization is consistent with the formation of SiAs-H bonds. From SIMS data, it is concluded that most of the hydrogen present in the passivated material is complexed with silicon. A microscopic model of the Si-H complex is proposed.