Thermoelectric power and electrical conductivity of LaAl0.95Mg0.05O3-delta and ErAl0.95Mg0.05O3-delta have been determined as a function of oxygen partial pressure between 1 and 10(-15) atm at 1000 degrees C and as a function of temperature in air and in a hydrogen-water buffer. In oxidizing atmospheres these compounds are found to be p-type conductors and in reducing atmospheres ionic conductors. The thermoelectric power at 1000 degrees C depends strongly on oxygen partial pressure. In both reducing and oxidizing atmospheres the thermoelectric power depends on temperature. In this work a new method for calculating ionic transference numbers directly from measurements of the thermoelectric power is discussed.