Thermal decomposition of the bulk and supported ammonium trioxalatometallate complexes (NH4)(3)[M(C2O4)(3)]center dot xH2O, where M=Cr, Fe, Ru in air has been studied using TG-DTA, TPR, and XRD. The decomposition temperatures and behavior depend on the metal nature. The decomposition of the supported Cr complex occurs below 340 degrees C, whereas it lasts up to 380 degrees C for the bulk phase, and results in the formation of CrO1.92 at 400 degrees C. First synthesized Ru trioxalato complex decomposed to RuO1.8 in three stages before 315 degrees C, the decomposition being completed around 280 degrees C if the complex is supported on titania. Both bulk and supported Fe complex decomposed rapidly at the same temperatures to FeOx with x = 1.37 for the supported system. The supported CrOx and FeOx phases are finely dispersed and X-ray amorphous. Their reduction with hydrogen below 500 degrees C resulted in Cr(Ill) and Fe(II) oxide species respectively. Only partial reduction to Fe-0 occurred at as high temperature as 850 degrees C. The RuO1.8/TiO2 system was reduced with hydrogen completely to Ru-0 below 300 degrees C. (C) 2009 Elsevier B.V. All rights reserved.