The relationship between oxide-ion conductivity and ordering of oxide ion in the (Y1-xLax)(2)(Ce1-xZrx)(2)O-7 system having a pyrochlore-type composition was investigated. The crystal phase of the system with a variable cation radius ratio, but with a constant lattice parameter, changed from a rare-earth-C (C)-type phase to a pyrochlore (P)-type phase through a fluorite (F)-type phase with increase in the composition (x), depending on the cation radius ratio r(A(3+))/r(B4+), which is a measure of ordering degree of oxygen vacancies. The oxide-ion conductivity increased once up to x = 0.15, and then decreased with increasing x. This change of the electrical conductivity was understood on the basis of the ordering degree of oxide ion. The Rietveld analyses of X-ray and neutron diffraction patterns, and Raman spectroscopy supported the ordering effect of oxide ion on the conductivity. (c) 2007 Elsevier B.V. All rights reserved.