Three compounds of formula (HB)2[M(dto)2], where HB+ is 2-amino-3-methylpyridinium, dto2- is 1,2-dithiooxalato-S,S’ ligand, and M is Ni(II), Pd(II), or Pt(II), have been synthesized and characterized by elemental analysis, and infrared and UV-Vis spectroscopy. The thermal behaviour of these complexes was studied by thermogravimetry (TG) and differential thermal analysis (DTA) under argon and argon-oxygen atmospheres. The final products of the thermal decomposition were examined by elemental analysis and X-ray powder diffractometry. The IR and UV-Vis spectra indicate the presence of the aromatic cations and the complex anions. The thermal analytical data show that the starting compounds and the surrounding atmosphere significantly influence the course of the decomposition reactions, as well as the final products. In an oxidative atmosphere, a mixture of nickel(II, III) oxides and sulphides, palladium(II) oxide and metallic platinum were obtained, whereas in an inert atmosphere, nickel sulphides, palladium(II) sulphide and platinum(0) were identified as the final products.