Starting from a general model for the defect structure of nickel and cobalt oxides, the concentrations and diffusion coefficients of the major defects in these oxides are determined as functions of temperature and oxygen pressure. At the higher oxygen pressures, the majority of cation defects are mononegative metal ion vacancies while at lower oxygen pressures appreciable concentrations of dinegative metal ion vacancies are present. It is shown that it is highly probable that the onion defects are dipositive oxygen vacancies. However, these are minority defects over the whole stability range of the oxides. It is not possible with the methods discussed here to determine their individual concentrations and diffusion coefficients. On the basis of this defect structure, a model for the oxidation of Co and Ni is proposed; the deductions from this model are compared with experimental results and good agreement is observed. It is shown that more than one type of defect contributes to the total transport through the oxide. For cobalt, these are mono- and dinegative metal ion vacancies; for nickel, dipositive oxygen vacancies also make a significant contribution.