A recently developed fast oxide ion conductor, namely lanthanum molybdenum oxide (La2Mo2O9, LAMO), was synthesized instantaneously by a citrate-nitrate auto-ignition process at a fixed citrate to nitrate ratio of 0.3 and characterized by thermal analysis, x-ray diffraction, impedance spectroscopy, and thermal expansion measurements. Crystalline LAMO has formed instantaneously during the combustion process. The signature of the order–disorder transition of stoichiometric LAMO around 570 °C was evident from differential thermal analysis, differential scanning calorimetry electrical conductivity, and thermal expansion measurements. Though the in situ x-ray measurements did not indicate any clear evidence of a phase transition, a stepwise change in the lattice parameter near the vicinity of the transition temperature was apparent thereby confirming the phase transition to be of first order in nature. The thermal expansion coefficient of LAMO was calculated to be 13.92 × 10−6/°K at 950 °C. The present method formed phase pure LAMO instantaneously and produced sintered samples with high conductivity, namely, 0.052 S/cm at 800 °C and 0.08 S/cm at 950 °C compared to LAMO prepared through various other solution routes.