The partial electronic conductivity of La0.9Sr0.1Ga0.8Mg0.2O3-delta has been measured as a function of temperature (T) and oxygen activity (a(O2)) in the ranges of 1073 less than or equal to T/K less than or equal to 1273 and 10(-1.5) less than or equal to a(O2) less than or equal to 10(-34.6), respectively, by using an ion-blocking, polarization technique. The p- and n-type conductivities may best be estimated as sigma (p)/S cm(-1) = (98 +/- 14)exp(- 1.12 +/- 0.12 eV/kT)(a(O2)(1/4)), sigma (n)/S cm(-1) = (1.8 +/- 0.2) X 10(8)exp(- 4.14 +/- 0.10 eV/kT)(a(O2)(-1/4)). Upon comparison with the literature, the p-type conductivity of LaGaO3-based oxides seems to increase as the amount of Sr with its activation energy little influenced. From the ionic and partial electronic conductivities, the electrolytic domain boundaries of La0.9Sr0.1Ga0.8Mg0.2O3-delta have been located: the lower boundary, e.g. at 1000 degreesC is 10(-23) atm of oxygen partial pressure.