The use Of Pe(x)Ce(1-x)O(2-delta) fluorites in electrochemical devices is hindered by several fundamental problems, one of which is the high thermal expansion coefficients (TECs), which have been shown to vary in the range (10-40) x 10(-6) K-1 between the temperatures 0 and 1000 degrees C. Thermogravimetric study shows that such a fluctuation of TECs, and non-linear lattice expansion on heating, are related to oxygen losses and can be controlled, to a substantial extent, by compositional selection. The influence of composition upon resultant mixed conductivity and oxygen permeability values is analysed for the compositions Zr0.1Ce0.9-xPrO2-delta and Ce1-xPrxO2-delta (x=0.2-0.3). Zr substitution leads to a decrease in total conductivity and an increase in average grain size. Oxygen permeability in these materials is controlled by both ambipolar bulk conductivity and oxygen surface exchange rates. Typical values of specific oxygen permeability are approximately 3 x 10(-8) mol/s cm at 1000 degrees C and compare well with other phases that are promising as oxide catalyst supports and precursors, such as La(Fe,Ni)O3-delta perovskites. A decrease in both ionic and electronic conductivities is observed with decreasing p(O-2) by measurements of oxygen concentration cell emf combined with impedance spectroscopy. Ion transference numbers show a positive temperature dependence, with typical values t(o)=0.8 at 950 degrees C under oxidizing conditions. (c) 2005 Elsevier B.V. All rights reserved.