Isothermal annealing studies of boron implanted silicon have been made at temperatures above 600°C for boron doses in the range of 1014to 1015ions/cm2. A 5 eV activation energy is obtained in the temperaturè range 600°C to 900°C. This value is the same as that associated with the generation and subsequent migration of vacancies in silicon. This indicates that the mechanism responsible for the increase in electrical activity of the implanted boron is due to the thermal generation of vacancies and the diffusion of these vacancies or vacancy complexes to interstitial boron atoms, which then become substitutional.