The thermal behaviour of metal compounds of the naturally occurring oxopurines hypoxanthine, xanthine and uric acid and of the synthetic pyrazolopyrimidines allopurinol and alloxanthine as well as of the thiopurine 6-mercaptopurine has been investigated using thermogravimetric and X-ray crystallographic techniques. The thermogravimetric data confirm the structural characteristics of the metal complexes. The degradation of hydrated complexes occurs in two steps with a dehydration reaction followed by complete decomposition to the corresponding metal oxides. Anhydrous compounds are decomposed in one step overall reactions. The temperature range of dehydrations of the hydrated complexes strongly depends on the binding mode of the water molecules. The reaction rate of the final decompositions of these complexes seems to be influenced by the respective metal ions. Copper and iron complexes show a sharp increase in the reaction rate in respect of the complexes of cobalt and nickel, whereas zinc, cadmium and manganese compounds are slowly decomposed over a wide temperature, range.