Controlled reactions have been undertaken between CO2 and polycrystalline YBa2Cu3O7‐σ in the temperature range 500°‐700°C. Typical reaction products are Y2BaCu05, BaC03 and CuO. Subsequent processing in O2 (e.g. sintering) results in the formation of liquid phases(s) by peritectic reaction at temperatures of 935‐975°C, well below the incongruent melting point of YBa2Cu3O7‐σ (∼1010°C). This “low temperature” liquid phase is shown to have a dramatic influence on the sintering process, and the resulting microstructures. The reactions are difficult to reverse even after extended times at temperatures >900°C. The results give an indication of the effect of adsorbed CO2 into YBa2Cu3O7‐σ powders, albeit on a more localized scale.