The thermal evolution of the crystal structure of the new fast oxide-ion conductor La2Mo2O9 across its Structural (X to [ first order transition at 580 degreesC has been followed using synchrotron X-ray diffraction (XRD). The influence on the transition, structure and oxide-ion conductivity of partial substitution by other rare earths (Nd, Gd, Y) is Studied. For Nd substitution the monoclinic alpha form is retained at room temperature in the whole compositional range. For Gd and Y substitutions, above a certain doping level, the Suppression of the phase transition and stabilization at room temperature of the cubic beta form are achieved. The cubic structures of La1.9Y0.1Mo2O9 at room temperature and of beta-La2Mo2O9 at 670 degreesC (revisited), both refined from neutron powder diffraction patterns, are very similar. Despite the cell volume reduction, lanthanide Substitution results above 500 degreesC in an increase of the anion conductivity compared to that of beta-La2Mo2O9, which might result from differences in samples' relative density. (C) 2003 Elsevier B.V. All rights reserved.