The goal of this study is twofold: to take a fresh look at the decomposition of calcium oxalate and to warn users of thermogravimetric analysis against the hasty interpretation of results obtained. Since the pioneer work of Duval 70 years ago, the scientific community has agreed unanimously as to the decomposition of anhydrous calcium oxalate (CaC2O4) into calcium carbonate (CaCO3) and CO gas, and that of the calcium carbonate into calcium oxide (CaO), and CO2 gas. We will demonstrate how these reactions, simple in appearance, in fact result from a succession of reactive phenomena involving numerous constituents both solid (CaCO3, free carbon) and gaseous (CO2 and CO) produced by intermediary reactions. The mass losses evaluated in the two distinct domains correspond closely to the molar masses of CO and CO2, respectively. The simple mathematical calculation of that mass loss has simply concealed the existence of other reactions, and, most particularly the Boudouard reaction and that of solid phases between CaCO3 and C. It just goes to show that appearances can be deceiving.