The evolution of in-fiber Bragg gratings' contrast and resonance wavelengths caused by molecular hydrogen loading and subsequent heat treatment in a hydrogen atmosphere at a pressure of 8 MPa is experimentally studied. Changes in the gratings' transmission spectra brought about by room-temperature hydrogen loading followed by isochronal annealing cycles at temperatures up to 700 degrees C and at invariable hydrogen pressure are recorded in situ. The gratings' thermal decay dynamics in hydrogen ambience and in air are found to be different and dependent on the gratings' type and on the type of glass in the fiber core. In light of the data obtained, the origin of type IIa gratings is anew under discussion.