Concurrent thermogravimetry (TG) and evolved‐gas analysis (EGA) were done for YBa2Cu3O7‐zand LaBa2Cu3‐O7‐zsuperconductors. The sample weights were monitored by thermobalance and the evolved O2and CO2species were monitored by quadruple mass spectrometer (QMS). No diffraction peak for the impurity phase containing a carbonate group was observed in the X‐ray diffraction patterns for these samples, but the release of CO2was detected by EGA. CO2gas began to evolve from YBa2Cu3O7‐zat 543°C and from LaBa2Cu3O7‐zat 692°C. Preparation of high‐quality YBa2Cu3O7‐zand LaBa2Cu3O7‐zsuperconductors is discussed on the basis of results of t