Two reversible reactions are involved in YBa2Cu3O6+x formation: a reaction between BaCO3 and CuO forming BaCuO2 and CO2, and a reaction of BaCuO2 with Y2O3 and CuO forming YBa2Cu3O6 which undergoes phase transformation to YBa2Cu3O6+x upon cooling. In-situ isothermal time resolved HT-XRD of a thin film was used to quantify the effect of CO2 on the kinetics of the first reaction. Increased CO2 partial pressure shifts the reactions to higher temperatures. At high CO2 partial pressure (>2 vol. %), the rate of the first reaction becomes essentially a step process with a very high activation energy. Noninstantaneous nucleation of the reaction products occurs at low CO2 partial pressure (0.5–1%) and temperatures (700°C). The data fit a 2-D diffusion-controlled mechanism with a zero nucleation rate for BaCO3 decomposition and a second-order nucleation rate for YBa2Cu3O6 formation. A comparison of the kinetics of a thin film (10 mm) as determined by HT-XRD with those of a thick sample (2 mm) determined by TG revealed that the transport of CO2 within the sample pores and to the ambient gas significantly affect the decomposition of BaCO3. For example, the formation of YBa2Cu3O6 in a thick precursor layer occurs in the 840 to 940°C range, exceeding by about 200°C that in which it is formed in thin films.