The temperature function of electrical conductivity of oxides and its activation energy is usually determined by impedance measurement of ceramic samples, exposed to a certain gas environment under temperature variation. By this isobaric mode of measurement, oxides with a range of oxygen-to-metal stoichiometry may release oxygen by thermal dissociation during heating. Thus, the temperature dependence of the conductivity cannot be assigned to an oxide of a fixed composition. In this work the pure temperature dependences of conductivity of samples of Pr0.8Sr0.2Mn0.8Co0.2O3-x and Sr0.85Ce0.15Fe0.5Co0.5O3-x perovskite-type oxides were determined by keeping constant the x values of the oxide during heating and cooling (isostoichiometric measurement). Conductivity versus temperature dependences of ionic/electronic mixed conductors were performed in this isostoichiometric manner to obtain the activation energy of a certain oxygen-to-metal (O/M) stoichiometry of the oxides. It could be demonstrated that the observed change of the temperature coefficient of conductivity from positive to negative is caused by the dominating influence of the oxygen loss by thermal dissociation on the value of conductivity. The activation energy of the isostoichiometric oxide is positive even in the temperature range of a negative temperature coefficient of conductivity.