Microcalorimetry was used to study the interaction between gaseous oxygen and NiO single crystals. The studies involved consecutive sorption of small and large oxygen doses at 1000 K. Fast and slow processes of oxygen incorporation into NiO were distinguished. The fast process involved oxygen sorption in the near-surface layer. Then oxygen incorporation was ratecontrolled by a slow lattice diffusion in the crystalline bulk. A power of oxygen pressure dependence was found (1/n = 1/4.25) which was consistent with the defect structure of NiO. Heat of formation of cation vacancies was determined [Δ_f=155 kJ/mole].