High ionic conductivity ceramics have potential technological applications in chemical sensors, ceramic permeable membranes, oxygen pumps, and solid oxide fuel cells. Recently ionic conductivity values as high as those of doped zirconia solid solutions have been found in a lanthanum molybdate compound. The high ionic conductivity of this compound, La2Mo2O9, is obtained at temperatures above the structural phase transition temperature (~580 °C). In this work the La2Mo2O9 ceramic material was prepared by the polymeric precursor technique and sintered at several dwell temperatures and soaking times to study the effect of sintering conditions on phase transition. It was found that there is a strong dependence of phase transition on the sintering profile. At 950 °C the phase transition is suppressed for short soaking times, whereas it is observed to occur for longer times. Moreover, the relative magnitude of conductivity is also dependent on the sintering conditions. The main conclusion is that the phase transition in La2Mo2O9 is particle sizedependent.