Defect properties of undoped serni-insulating and silicon doped n-type GaAs annealed Lit different arsenic vapor pressures have been studied by means of positron annihilation and Hall measurements. In both types of samples, formation of monovacancy-like defects during annealing was observed. On increasing the arsenic pressure, the concentration of these defects increases in GaAs:Si and decreases in undoped GaAs. In GaAs:Si, the defect was identified LIS Si(Ga)-V(Ga) complex. In undoped GaAs, arsenic vacancies are formed, which are part of the defect complex.