Thermal decomposition of the oxalates of zinc, nickel and iron(II) have been reexamined from a fresh experimental approach. Differential thermal analysis (DTA) and thermogravimetry (TG) of the individual oxalates and of mixtures of zinc oxalate with either nickel/iron oxalate or products of decomposition of the latter two, were carried out in air in sample cells made of different materials (Pt, Al, Al2O3, Ni). The information gathered from thermoanalytical experiments, together with information derived from specific chemical tests for the evolution of carbon monoxide during decomposition, chemical analyses, XRD and stoichiometric and thermochemical considerations helped to specify some of the inadequately explained features of the courses and kinetics of decomposition of the metal oxalates. Demonstrative evidence strongly indicates that secondary exothermic oxidation of carbon monoxide liberated in situ, takes place at relatively low temperatures in the presence of Pt (sample cell material), NiO and Fe3O4 (decomposition products) which act as catalysts and are reflected in overall exothermic DTA responses that are endothermic in air/oxygen in the absence of catalyst surfaces.