DSC, TG and TG-FT-IR, and XRPD have been used to examine the effects of supposedly inert atmospheres of argon and nitrogen on the mechanism of the thermal decomposition of copper(II) oxalate. The DSC curves in pure argon at 10 degrees C min(-1) show a broad endotherm with onset at about 280 degrees C and maximum at about 295 degrees C. In mixtures of argon and nitrogen, as the proportion of argon gas is decreased, the endothermic character of the decomposition decreases until, when nitrogen is the main component, the decomposition exhibits a complex broad exothermic character. XRPD studies showed that, regardless of the proportions of nitrogen and argon, the DSC residues consisted of mainly copper metal with small amounts of copper(I) oxide (cuprite) and, under some conditions, traces of copper(II) oxide (tenorite). Various explanations for this behaviour are discussed and a possible answer lies in the disproportionation of CO2(g) to form small quantities of O-2(g) or monatomic oxygen. The possibility exists that the exothermicity in nitrogen could be explained by reaction of the nitrogen with atomic oxygen to form N2O(g), but this product could not be detected using TG-FT-IR. (c) 2006 Elsevier B.V. All rights reserved.