The effect of temperature and oxygen partial pressure on the electrical conductivity of simple, lithium and titanium doped MgO was investigated in the temperature range 873-1173 K. Pure and lithium doped MgO exhibited always p-type conductivity in the temperature range investigated whereas titanium doped MgO had always n-type conductivity. Defect chemistry equations are used to describe the formation mechanism of specific lattice defects. It is proved that lithium doping increases the concentration of oxygen vacancies and hence the p-type conduction whereas titanium doping depresses the formation of oxygen vacancies. Some of the known properties of these oxides, such as catalytic ones, are discussed by using defect chemistry concepts.