The thermal decomposition of ferrous fumarate half hydrate, FeC4H2O4 . 0.5H2O and ferrous tartrate one and a half hydrate, FeC4H4O6 . 1.5H2O have been studied by two probe direct current electrical conductivity measurements under the atmospheres of static air and dynamic dry nitrogen. The products at each decomposition stage have been characterized by infrared spectroscopy, X-ray powder diffraction Mossbauer spectroscopy and gas chromatography. The change in the oxidation state of iron ion is clearly followed through the course of decomposition via the changes in the isomer shift and quadrupole splittings. The formation of intermediates such as FeO, Fe3O4, gamma-Fe2O3 and alpha-Fe2O3 are also discussed and the results of the Mossbauer measurements are correlated with the thermal analysis of these dicarboxylates.