Using ion implantation techniques, we have studied the trapping, detrapping, and diffusion of H in SiO2 containing a high density of defects and a high concentration of excess Si. In SiO2 sample implanted with Si and H, the implanted H moves toward the surface after stopping at the projected range, and is trapped by vacancies and silicon dangling bonds. If the Si-implanted SiO2 is annealed to 1423 K prior to H implantation, the trap sites changes from the Si–H bonds in the unannealed sample to the dangling bonds at the nanocrystalline interfaces. The dependence of retention and trapping site of hydrogen on the Si implantation showed that, irrespective of the preannealing, the retention of hydrogen rapidly increases above a dose of 1×1017 Si/cm2 and then levels off, and the trapping site for 6×1017 Si/cm2 is distinctly different from that for 3×1017 Si/cm2. A possible reason for the latter result is that, in the case of the unannealed sample, the implanted silicon atoms coagulate to form clusters at 6×1017 Si/cm2 and hydrogen is trapped by the defects inside the silicon clusters. In the case of the annealed sample, the volume fraction of the silicon nanocrystals becomes large for the 6×1017 Si/cm2 implantation and, as a result, the fraction of the V–H complex inside the nanocrystals increases. © 2004 American Institute of Physics.