The effect of aliovalent dopants on the defect chemistry of metal oxides is reviewed. The discussion focuses on the stoichiometric solid solution obtained by combination of the stoichiometric dopant oxide and the stoichiometric host oxide without gain or loss of oxygen. The charge of the dopant centers is then compensated solely by ionic defects. As the oxygen activity is raised above that in equilibrium with the stoichiometric composition the lattice defects are gradually replaced by holes. Conversely, when the oxygen activity is reduced below that in equilibrium with the stoichiometric composition the lattice defects are gradually replaced by electrons. The near-stoichiometric region of a pure metal oxide is replaced by two regions with charge compensation by a product of reduction, e.g. oxygen vacancies or electrons, in the region of lower oxygen activity, and by a product of oxidation, e.g. cation vacancies or holes, in the region of higher oxygen activity.