Amorphous Si has been investigated by variable-energy positron annihilation spectroscopy (PAS) and lifetime measurements of optically generated free carriers. The density of positron-trapping defects can be reduced by thermal annealing at 500°C. Simultaneously, the density of bandgap states is reduced as indicated by an increased photocarrier lifetime. Hydrogen, implanted and annealed at 150°C, leads to an increased photocarrier lifetime and reduced positron trapping. It appears 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 amorphous Si.