We studied the properties of irradiation-induced point defects and thermal equilibrium vacancies in Si by detecting hydrogen interacting with these point defects. First, we show the irradiation temperature dependence of the point defect concentration. We studied the optical absorption spectra of complexes of hydrogen and point defects generated by electron irradiation of hydrogenated Si crystals at low temperatures. The concentrations of complexes of hydrogen with vacancy and Frenkel pairs showed a non-monotonic dependence on irradiation temperature. Hence, a metastable interstitial-vacancy (Frenkel pair) model is not applicable to electronirradiated Si. From the dependences of the intensity ratios of the optical absorption due to complexes of H and 2 point defects, we determined the migration energies of self-interstitial and Frenkel pairs relative to that of the vacancy. The energies were similar. Second, we determined the vacancy formation energy in high-purity Si by a new quenching method: specimens were heated in H gas at various temperatures followed by quenching in 2 water. The formation energy determined by the above method was 4.0 eV, which agrees well with recent theoretical values.