The kinetics of E ' centres induced in silica by 4.7 eV laser irradiation was investigated observing in situ their optical absorption band at 5.8 eV. After exposure, the defects decay due to reaction with diffusing molecular hydrogen of radiolytic origin. Hydrogen-related annealing is also active during exposure and competes with the photo-induced generation of the centres until a saturation is reached. The concentrations of E ' and H(2) at saturation are proportional, so indicating that the UV-induced generation processes of the two species are correlated. These results are consistent with a model in which E ' and hydrogen are generated from a common precursor Si-H.