Thermal decomposition of Fc4Sn3Br166H2O was investigated by thermogravimetry. The decomposition products were characterized by U.V.-Visible spectroscopy, chemical analysis and X-ray diffraction. The TG curve of the complex showed three decomposition steps. In the first, a mixture of HBr and H2O was evolved. A ferricinium tin bromide compound (Fc4SnBr8) was found to sublime in the second step, whereas a mixture of HBr and H2O was given off in the third, leaving SnO2 as the residue, The stoichiometry of the thermal decomposition of [Fc4Sn3Br16 6H2O] has been established and a possible mechanism for its decomposition is suggested.