The elaborated and well characterised YBa 2 Cu 3 O 7 - x and PrBa 2 Cu 3 O 7 - x polycrystals were studied by means of differential scanning calorimetry and thermogravimetry in different atmospheres. The YBa 2 Cu 3 O 7 - x polycrystals were also studied by high temperature x-ray diffraction. It is found that at about 300 °C the polycrystal sample of the YBa 2 Cu 3 O 7 - x shows an endothermal anomaly and a weight loss. But the sample of the PrBa 2 Cu 3 O 7 - 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 YBa 2 Cu 3 O 7 - x jump remarkably at about 300 °C. Comparing the behaviours of the YBa 2 Cu 3 O 7 - x and the PrBa 2 Cu 3 O 7 - 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.