Influence of low-temperature annealing on electrical and luminescent properties of semi-insulating n-CdTe:Sn is studied. It is shown that annealing at T ≥ 453 K modifies a near-surface layer of the crystals and this enhances the hole component of the conductivity. It is explained by Cd evaporation and subsequent enrichment of the near-surface layer with cadmium vacancies. Results of electrical measurements are confirmed by our low-temperature photoluminescence experiments.