The oxygen ion transference numbers of La1.7Bi0.3Mo2O9, La2Mo1.7W0.3O9 and La2Mo1.95V0.05O9 ceramics, determined by modified faradaic efficiency and e.m.f. methods at 973-1173 K, vary in the range 0.995-0.977 in air, decreasing when temperature increases. The activation energies for the ionic and electronic transport are 61-71 kJ/ mol and 123-141 kJ/mol, respectively. Reducing oxygen chemical potential leads to increasing n-type electronic contribution to the total conductivity, which remains, however, essentially p(O-2)-independent down to oxygen pressures of 10(-4)-10(-3) atm and exhibits reversible drop on further reduction, probably due to phase decomposition. Doping La2Mo2O9 with calcium results in segregation of a CaMoO4-based phase, accompanied with increasing electronic transport. The average thermal expansion coefficients of La2Mo2O9-based materials, calculated from dilatometric data in air, are (14.4-14.8) x 10(-6) K-1 at 300-700 K and (16.4-22.5) x 10(-6) K-1 at 700-1070 K. (C) 2004 Elsevier Ltd. All rights reserved.