FTIR spectroscopy measurements indicate a presence of high concentration hydrogen existing in the liquid encapsulated Czochralski grown undoped InP wafers. These undoped InP can be annealed to be semi-insulating (SI) reproducibly. Hydrogen can facilitate the formation of anti-site defects in InP and can also induce electrically-active hydrogen related defects. The annealed behavior of InP is proved possible to be originated from the neutralization effect of hydrogen that is supplied by the dissociation of hydrogen complexes in the process of annealing. The process of hydrogen passivation of donors is discussed. Besides neutralizing impurities and defects in InP, hydrogen induces new hydrogen related defects in the band gap, and can also activate the formation of complexes involving p(In), so it plays an important role in the charge compensation process in SI InP obtained by high temperature annealing.