The formation of YBa2Cu3O6+x in air involves two main reactions. The first is between BaCO3 and CuO to form BaCuO2 and CO2. Increasing the partial pressure of carbon dioxide in air shifts the endothermic BaCO3 reaction to higher temperatures. At a fixed carbon-dioxide concentration in air, the reaction onset-temperature of a thick sample (2 mm) determined by TG/DTA exceeds by approximately 200 degrees C that of a thin film (10 mm) determined by in situ HT-XRD. Evolved CO2 in thick samples disguises the dependence of the intrinsic reaction onset-temperature and reaction rate on the ambient CO2 concentrations. The intrinsic equilibrium constant and associated thermodynamic parameters for this reaction should be determined by experiments in which the impact of transport limitations is eliminated, i.e., use of a thin film and high gas-velocity next to the sample surface.