The structure of pure amorphous Si, prepared by ion implantation, has been investigated by variable‐energy positron annihilation spectroscopy (PAS) and lifetime measurements of optically generated free carriers. In general, PAS measurements are thought to be sensitive to vacancy‐type defects while the carrier lifetime depends on the density of band‐gap states (e.g., dangling bonds). The PAS measurements indicate that the density of positron‐trapping defects can be reduced by thermal annealing at 500 °C. Concurrent with the removal of structural defects the density of band gap states is reduced as indicated by an increased photocarrier lifetime by a factor of 10. Some material has been implanted with H+and annealed at a low temperature (150 °C). The hydrogen is expected to passivate electrical defects associated with strained and dangling bonds and indeed the photocarrier lifetime is increased in this material. Moreover, the PAS measurements cannot distinguish this material from 500 °C annealed amorphous Si, indicating that (some of) the electrical defects are associated with positron‐trapping, and therefore possibly vacancy‐type, structural defects. Finally, both methods have been used to detect small amounts of ion irradiation damage in annealed amorphous Si.