Thermal decompositions of three 4-aminopyridinium salts of square-planar inorganic metal(II) 1,2-dithiooxalato-S,S’ complexes, (HC5H6N2)2[M(S2C2O2)2] . 2H2O where M is Ni(II), Pd(II) and Pt(II) (hereafter abbreviated as NIDT4AP, PDDT4AP and PTDT4AP, respectively) have been studied by thermogravimetry (TG and DTG) and differential thermal analysis (DTA) under argon-oxygen and argon atmospheres. Thermoanalytical data show that the surrounding atmosphere influences the thermal decomposition processes as well as the final products. In argon-oxygen atmosphere, the final residues were identified as a mixture of nickel sulphides and oxides for NIDT4AP, metallic palladium and palladium oxide for PDDT4AP, and platinum(0) for PTDT4AP. Thermal decompositions in an inert atmosphere yielded nickel(II) sulphide for NIDT4AP, a mixture of Pd/PdS for PDDT4AP, and metallic platinum for PTDT4AP. The final decomposition products were identified by elemental analysis and the X-ray powder diffraction technique. No other stable intermediate products, except the corresponding anhydrous compounds, were found during the thermal decomposition owing to the complexity and the overlap of the processes in oxidative and inert atmospheres.