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The elaborated and well characterised YBa2Cu3O7−x and PrBa2Cu3O7−x polycrystals were studied by means of differential scanning calorimetry and thermogravimetry in different atmospheres. The YBa2Cu3O7−x polycrystals were also studied by high temperature x-ray diffraction. It is found that at about 300 °C the polycrystal sample of the YBa2Cu3O7−x shows a endothermal anomaly and a weight loss. But the sample of the PrBa2Cu3O7−x shows a completely opposite behaviour, namely, an exothermal anomaly and an increase of weight. High temperature x-ray diffraction demonstrates that the lattice parameters of the YBa2Cu3O7−x jump remarkably at about 300 °C. Comparing the behaviours of the YBa2Cu3O7−x and the PrBa2Cu3O7−x at about 300 °C, an opposite behaviour of the superconductors and the semiconductors is observed. We try to attribute this different behaviour to an unknown phase transition and a thermal instability, and believe this result may be of importance to understand the different properties between the superconducting and the semiconducting samples.