Thermal decomposition of FeC2O4. 2H(2)O under Ar, H-2/Ar mixture and H-2, was studied by thermogravimetric measurements with quantitative analysis of the gaseous products by both mass spectrometry and infrared spectroscopy. The oxalate decomposition appears rather independent of the Hz partial pressure, giving a mixture of Fe3O4, alpha-Fe, CO and CO2. But, in fact, when the Hz partial pressure is high enough, CO produced by the decomposition is hydrogenated in CH4 (and in less extent into C2H6) according to a Fischer-Tropsch reaction catalyzed by alpha-Fe. This reaction is also accompanied by the formation of cementite (theta-Fe3C). Upon further heating, Fe3O4 reduction to metallic iron occurs followed by decomposition of iron carbide also into alpha-Fe. The effect of experimental parameters on the reaction process is discussed.