Thermal anneal activation of defects resulting from atomic hydrogen treatment of Si wafers in an electron cyclotron resonance (ECR) plasma system have been studied. Following short-term (4-12 min), low-temperature hydrogenation, n- and p-Si wafers were annealed over the temperature range 300-750°C for 20 min. While only a small broad peak is seen immediately after hydrogenation, several pronounced and distinct majority carrier trap levels show up in deep level transient spectroscopy (DLTS) measurements on samples annealed at 450°C and above. The concentrations of these deep levels reach a maximum at anneal temperatures around 500°C and drop substantially beyond 750°C. This phenomenon appears to be unrelated to the presence of oxygen in Si and is of potential importance in silicon processing technology.