Oxide ion conduction in A(3+)B(3+)O(3)-type perovskites, (La0.9Sr0.1)(MO3-delta)-O-III (M-III = Al, Ga, Sc, In, and Lu) has been investigated as a function of crystallographic free volume (V-f), taking the distortion from ’ideal’ cubic structure into consideration. The Ga compound, whose V-f is ca. 13 Angstrom(3), showed the maximum conductivity of 8.1 x 10(-2) S cm(-1) (at 980 degrees C, in N-2) and the minimum activation energy of 0.60 eV (from 730-980 degrees C, in N-2) for oxide ion conduction. This behavior arises from the balance between an increase in V-f and a decrease in symmetry in the perovskite structure.