The release of atomic H during charge injection into SiO 2 layers thermally grown on silicon was monitored by measuring the H-induced decrease in the concentration of active boron acceptors at the Si surface. It is found that H is set free both upon trapping of holes in the oxide and upon their neutralization by electron injection. The H + release upon hole trapping correlates with the generation of paramagnetic E' centers in the oxide (O 3 =Si defects) unveiling the hole trapping on Si-H bonds as the primary H liberation mechanism in SiO 2 . The additional H liberation upon neutralization of the positive charge created by hole injection suggests that a large portion of this charge is due to protons trapped in the oxide, which, upon electron trapping, are released as atomic H.