Gravimetric and Seebeck‐effect measurements have been made on CoO single crystals doped with Li in the temperature range of 900°–1200°C and partial pressures of oxygen up to 1 atm.For low‐doped crystals and high oxygen pressures, the gravimetric and Seebeck‐effect results confirm a simple model by which the positive‐charge carriers are given at equilibrium by the singly ionized vacancies and the Li ions. By this model the concentration of holes is increased, and the concentration of vacancies is decreased by the addition of Li.The results at low pressures of oxygen for the low‐doped samples as well as the results in all the range of oxygen pressures for the heavier‐doped samples show that the model has to be enlarged, taking into account the contribution of Co interstitials to the defect structure of CoO(Li) and that of electrons to the conductivity in CoO(Li).