The effect of pO(2) on the electrical conductivity of La0.5Sr0.5Ga0.2Fe0.8O3-delta was investigated at 750 < T < 1050 degrees C and 10(-17)< pO(2)< 0.5 atm. At T < 850 degrees C, a discontinuity, i.e., a plateau and a steep slope, was observed in the intermediate pO(2) region, indicating that a phase separation occurred, most likely forming a mixture of a perovskite and an oxygen vacancy ordered phase. Above 900 degrees C, a smooth isotherm without the discontinuity was obtained. The slopes in the plot of the logarithm of the conductivity (sigma) vs log pO(2) were approximate to 1/4 in the p-type pO(2) region. The pO(2) dependence of the oxygen nonstoichiometry (delta) was studied using the solid-state coulometric titration technique. The phase separation was also observed in the form of the delta isotherm hysteresis at the condition where the discontinuity appeared in the conductivity isotherms. Based on the nonstoichiometry data, a point defect model was derived and the partial molar quantities (mu(O2), h(O2), and S-O2) of oxygen in La0.5Sr0.5Ga0.2Fe0.8O3-delta were calculated.