Total electrical conductivity of (Ba0.5+xLa0.5-x)(2)(In1-xMx)(2)O-5.5 (M=Zr and Cc) and (Ba0.5-xLa0.5+x)(2)(In1-xMx')(2)O-5.5 (M'=Mg, Ca and Sr) solid-solution systems was measured as functions of temperature and oxygen partial pressure. The oxide-ion conductivity of all the solid-solution systems increased at first with an increase in the unit cell free volume. However, it showed a maximum value at a unit cell free volume and then decreased. All the solid-solution systems showed a similar dependence of the oxide-ion conductivity on the unit cell free volume, even if they have different average ionic valences in the A-and B-sites. From the results of the Raman spectra measurement, the decrease of conductivity in the larger unit cell free volume range is presumed to arise from the local distortion of oxygen octahedra. (C) 2004 Elsevier B.V. All rights reserved.