The electrical conductivity of NiO was measured at 740°C in an oxygen pressure range of 10^-2-1.3 Torr. By means of continuous recording, longtime experiments were performed. The results show that for any admittance of oxygen, the electrical conductivity initially increased and then decreased to its initial value. For pressures higher than 0.1 Torr the decrease of the signal was reduced and the time required to attain the initial value sometimes reached several days. These results suggest that the electrical conductivity changes may be considered as a transitory phenomenon connected to attaining gas-solid equilibrium.