Thermal behaviour of five or six-coordinate rhodium and iridium complexes of two tripodal phosphines, CH3C(CH2PPh2)(3) and CH3C(CK2P(m-CF3C6H4)(2))(3), and their mixed-ligand complexes containing Cl, CO and MeCN was studied by TG, DTA and DTG in dynamic nitrogen atmosphere. The plain complexes exhibit very high thermal stability and degradation of the phosphine ligands follow a complicated process, while the first stage of the mixed-ligand complexes corresponds to removal of CO or MeCN and decomposition of the phosphine ligands occurs in the subsequent stages. All complexes undergo complete decomposition to form the respective metals as the final decomposition product. It was found that iridium complexes showed much higher thermal stability than their rhodium analogues.