The passivation by diffusing H 2 of silicon dangling bond defects (E' centers, O 3 ≡Si•)induced by laser irradiation in amorphous SiO 2 (silica), is investigated in situ at several temperatures. It is found that the reaction between E' center and H 2 requires an activation energy of 0.38eV, and that its kinetics is not diffusion-limited. The results are compared with previous findings on the other fundamental paramagnetic point defect in silica, the non bridging oxygen hole center, which features completely different reaction properties with H 2 . Besides, a comparison is proposed with literature data on the reaction properties of surface E' centers, of E' centers embedded in silica films, and with theoretical calculations. In particular, the close agreement with the reaction properties of surface E' centers with H 2 leads to conclude that the bulk and surface E' varieties are indistinguishable from their reaction properties with molecular hydrogen.