Medium energy ion scattering (MEIS) was used to study distribution of ion-implanted Sb dopant in Si with excess vacancies and SIMOX substrates and the effects of thermal treatments. Extra vacancies in Si were generated by N or O pre-implantations at high temperature. Effects related to the different chemical nature of the pre-implanted species are expected under these conditions. Different Sb annealing behavior and distribution were observed for O and N pre-implanted Si. The oxygen containing samples (SIMOX and O pre-implanted Si) presented higher substitutionality after long annealing times. The nitrogen pre-implanted Si presented the lowest amount of segregated Sb and more uniform dopant distribution. Both pre-implanted samples, N and O, had a large dopant loss to the atmosphere during annealing.