An angle-resolved X-ray photoelectron spectroscopy (ARXPS) method is first introduced for elucidation of the mechanism of Te substitution by Se in Sb2Te3 crystals, The obtained results are consistent with the substitution of the selenium atoms predominantly into the Te-2 atomic layers. The observed substitution makes it possible to evaluate the interactions of incorporated Se atoms with antisite defects, Sb-Te’. The incorporation of Se atoms into the lattice increases ionicity of crystal bonds and suppresses the Sb-Te’ defect concentration, which is accompanied by a decrease in hole concentration.