Hydrogen evolution transients were measured for hydrogenated amorphous silicon prepared by Si implantation of crystalline silicon and subsequent hydrogen implantation. The evolution curves are found to be similar for different H concentrations but with entirely different atomic and nanoscale structures, as was evidenced by small-angle x-ray scattering and infrared absorption investigations [Phys. Rev. B53, 4415 (1996)]. This behavior is explained by a hydrogen-diffusion controlled effusion with a limited density of sites in the amorphous material that can be occupied by hydrogen. The experimental effusion curves are modeled by using diffusion coefficients in the implanted layers that were determined by secondary-ion mass spectrometry. Diffusion through a highly disordered material of low H content is found to have an activation energy of 2.26 eV. ©1997 American Institute of Physics.