The guanidinium hexafluorometallates are very stable compounds because of the formation of NH...F bonds in the structure. The compounds form series with isotype structures that depend on the charge of the central atom : [C(NH2)3]3MF6 (cubic), [C(NH2)3]2MF6 (monoclinic), [C(NH2)3]MF6 (polymorphic). The thermal decomposition of 19 guanidinium hexafluorometallates has been investigated under quasi-static conditions (DTA, TG and DTG). In this way, hydrolysis reactions were repressed and a correlation of the obtained results with thermodynamic data became possible. The solid and gaseous reaction products were characterized by X-ray analysis, IR spectroscopy and mass spectroscopy. The results are discussed with the help of oxidation state diagrams.