Defect properties of undoped semiinsulating and silicon doped n-type GaAs annealed at different arsenic vapor pressures have been studied by means of positron annihilation and Hall effect measurements. In both types of samples, formation of monovacancylike defects during annealing was observed. The concentration of these defects increases in GaAs:Si and decreases in undoped GaAs when the arsenic pressure increases. In GaAs:Si, the defect was earlier identified as Si_Ga-V_Ga complex, however, in undoped GaAs arsenic vacancies are formed, which are part of defect complex.