Unusual structural phenomena have been observed through high temperature X-ray diffraction in fast oxide-ion conductor La1.92Ca0.08Mo2O8.96. A demixion process occurs above 640 degrees C, with calcium segregation leading to the formation of CaMoO4 impurity. It recombines at higher temperature to reform the pure cubic LAMOX phase (beta-type disordered form). In addition, at lower temperature, a double reversed beta/alpha/beta phase transition (where alpha is the monoclinic ordered form) is observed. It is due to the metastability of the beta-type structure at low temperature. Depending on the thermal history, La1.92Ca0.08Mo2O8.96 can therefore be obtained in either its alpha or its beta form at room temperature. Sample shaping as a pellet with high relative density suppresses the low temperature phase transitions and stabilizes the cubic beta form. (c) 2006 Elsevier B.V. All rights reserved.