The motion of hydrogen and deuterium in undoped multilayers of rf sputter deposited a-Si:H, was recently studied by IR absorption and SIMS profiling of the annealed films. It was shown that in films containing significant SiH2 and SiH3 bonding configurations, as determined by IR absorption spectra and usually associated with microvoids, the connected network providing the H diffusion path is disrupted below the percolation threshold. Measurements of the index of refraction of these films were shown to be indeed consistent with this assignment of SiH2 and SiH3 configurations. In films containing essentially only isolated monohydride type bonding, the smearing of the interfaces exhibited a power law time dependence of the diffusion constant, D ∝ t-∝. Yet the value of ∝, 0.75±0.1, is considerably higher than that found by Kakalios, Street, and coworkers on p- and n-doped a-Si:H films deposited by rf glow discharge decomposition of silane and correlated with electronic transport decay processes. The results thus introduce a complicating factor in the suggested correlation between the hydrogen motion and the formation and annealing of light induced defects which degrade the photoconductivity of these films. The diffusion in a-Ge:H appears to be qualitatively similar, but considerably faster. Yet annealing processes in a-Ge:H apparently introduce additional complicating factors which may be related to the poor device quality of a-Ge:H based alloys.