We discuss a possible mechanism leading to the dimerization of hydrogen, produced by irradiation in OH-containing a-SiO2. We describe the hydrogen-annealing behavior by combining both fractal and classical kinetics to account for the H-0 + H-0 --> H-2 process. The proposal for the combined kinetics is based on a possible picture of the a-SiO2 atomic network where the lattice is seen not as a continuous but instead as a ''granular'' random lattice as indicated also by Raman-scattering experiments. Many experimental data, previously reported by Tsai et al., concerning electron-spin-resonance measurements of the radiolytic-hydrogen-annealing behavior in a-SiO2 are well reproduced by the model. The physical meaning of the parameter values utilized in the fitting procedure is discussed. Finally it is suggested that the analysis of the anomalous diffusion may help in understanding the atomic structure in amorphous solids.