A model is proposed for the kinetics of photoinduced defects and the photorefractivity in germanosilicate fibers. It is based on the existence of short lived large energy fluctuations which produce transient traps for carriers that release their energy and enhance defects creation. The enhancement of the photorefractivity with the presence of hydrogen is explained as a result of two mechanisms, that involve the increase in the number of weak bonds and the stabilization of the created defects. In some special cases analytic solutions are found that explain reasonably well the experiments. ©1995 American Institute of Physics.