Oxide ion conduction properties of La1-xGdxGa0.8Al0.1Mg0.1O2.95 were discussed in terms of tolerance factor and free volume. The oxide ion conductivities increased with Gd content and attained the maximum at x = 0.10. Enhancement in oxide ion conduction up to x = 0.10 was attributable to the approach to an optimum tolerance factor, which is reportedly around 0.96, whereas degradation above x = 0.10 was considered to result from the reduction of free volume caused by replacement of smaller Gd for La. (c) 2006 Elsevier B.V. All rights reserved.