The correct thermal decomposition mechanism of Cu(C 4H 4O 6) · 3 H 2O and Co(C 4H 4O 6) · 2.5 H 2O was established using simultaneous thermal analysis (TG, DTG, DTA) and mass spectrometry (MS) under inert gas atmosphere (Ar). The reaction mechanism was found to be endothermic and radical in nature for the M(C 4H 4O 6) decomposition. Mass spectrometry proved the primary gaseous decomposition products to be H 2O and C 4H 4O 2− 6, the tartrate diradical. This diradical itself showed a characteristic degradation scheme towards smaller and more stable molecules like H 2O, CO 2 and CO. The formation of the metals Cu and Co could be proved by X-ray and TG experiments.