We show that the detailed atomic structure of vacancy–impurity complexes in Si can be experimentally determined by combining positron lifetime and electron momentum distribution measurements. The vacancies complexed with a single impurity, V–P and V–As, are identified in electron irradiated Si. The formation of native vacancy defects is observed in highly As-doped Si at the doping level of 10 20 cm −3 . The defects are identified as monovacancies surrounded by three As atoms. The formation of V–As 3 complex is consistent with the theoretical descriptions of As diffusion and electrical deactivation in highly As doped Si.