A new series of doped-Ruddlesden-Popper phases, of general formula Sr(3)M(2-x)M(x)’O-7-y (M = Ti, Zr, M’ = Al, Ga, In) were synthesized and their conductivity was investigated as a function of temperature and oxygen partial pressure. The highest total conductivity in air is 1.6 x 10(-3) S/cm at 950 degrees C for both Sr3Ti1.75Ga0.25O6.875 and Sr3Ti1.9In0.1O6.95. Measurements of the conductivity as a function of oxygen partial pressure indicate that p-type electronic conductivity predominates at high p(O-2), while a p(O-2)-independent region at intermediate p(O-2) values indicates ionic conductivity. The magnitude of the ionic conductivity at 700 degrees C is approximately 10(-5) S/cm.