The present paper reports the first stage of the study by the present group on the thermal decomposition of La2Mo2O9 under lower oxygen partial pressure. The stability of La2Mo2O9 at 1473 K in an oxygen partial pressure (pO(2)) ranges from 2.44 x 10(-5) to 4.33 x 10(-9) Pa was studied by XRD. The results showed that La2Mo2O9 could remain stable in a p(O2) range greater than 2.44 x 10(-5) Pa. While below this pO(2), the formation of several reduced and crystallized molybdates were found. By means of XRD, TEM, SAED, and EDS, the decomposition process of La2Mo2O9 at 1473 K under p(O2) of 9.75 x 10(-7) Pa was investigated. The intermediate and final products formed during the thermal decomposition were characterized. It was found that the La2Mo2O9 decomposition consisted of two steps. La2Mo2O9 first decomposed to intermediate products like LaMo2O5, La5Mo4O14, and La12Mo6O35 and then to final products like La5Mo3O16 and a new Mo-rich compound. Tilt series of SAED patterns of this new structure were recorded. These measurements were used for the reconstruction of the reciprocal space. The cell attained was monoclinic with parameters: a approximate to 1.195 nm, b approximate to 4.133 nm, c approximate to 1.314 nm, and beta approximate to 95 degrees.