The Rosenburg method has been used to investigate the deviation from stoichiometry and the defect diffusion coefficient in NiO between 1200 and 1300 degrees C and in the oxygen pressure range 0.05-0.25 atmospheres. Simple point defect chemistry has been applied to interpret the oxygen pressure and temperature dependence of the data. The formation enthalpy and entropy of nickel vacancies at constant oxygen pressure have been estimated to be 75 kJ/mol and - 15 J/mol K, respectively. The defect diffusion coefficient is essentially independent of the oxygen pressure and the migration enthalpy is determined to be approximately 150 kJ/mol. Combination of the enthalpies for formation and migration of the defects yields an activation enthalpy of 225 kJ/mol for Ni self diffusion. From the oxygen pressure dependence of the nonstoichiometry, singly charged nickel vacancies have been concluded to be the predominating point defect.