We investigated the effect of the addition of Sb atoms on the properties of defect states of pure amorphous selenium. The trap levels have been studied by a combination of conventional thermally stimulated depolarization current spectroscopy and time-of-flight measurements. Taken together, these experiments show that Sb introduces a new set of shallow traps for electrons at energies 0.22, 0.34 and 0.45 eV below the conduction band edge, in addition to that existing in pure selenium. At the same time, progressive addition of antimony markedly enhances the deep trapping of holes.