Simultaneous thermogravimetric/differential thermal analysis of Gd2Mo3O12 showed an irreversible phase transition at 1178 K whereas Gd2W3O12 showed reversible phase transition at 1433 K, which were confirmed by powder X-ray diffraction. The thermal expansion behavior of alpha-Gd2Mo3O12 (room temperature phase), beta-Gd2Mo3O12 (phase obtained by heating Gd2Mo3O12 at 1223 K) and Gd2W3O12 have been investigated using high temperature X-ray diffractometer. The cell volume of alpha-Gd2Mo3O12, beta-Gd2Mo3O12 and Gd2W3O12, fit into polynomial expression with respect to temperature, showed positive thermal expansion up to 1073,1173 and 1173 K, respectively. The average volume expansion coefficients for alpha-Gd2Mo3O12, beta-Gd2Mo3O12 and Gd2W3O12 are 39.52 x 10(-6), 21.23 x 10(-6) and 37.96 x 10(-6) K-1, respectively. (C) 2008 Elsevier Ltd. All rights reserved.