This paper presents thermoanalytical features (DTA and TG) for YBa2Cu3O7-x. It was shown that deoxidation occurred from about 400 degrees C for the pure substance and that at 1020 degrees C it was decomposed peritectically, forming Y2BaCuO5, which then decomposed similarly at 1247 degrees C to Y2O3. Both reactions were accompanied by endothermic effects and a jump-type decrease in weight. Occurrences of additional effects, especially different DTA and TG courses, were caused by the presence of strange phases. This inconvenience was mostly due to secondary reactions, for instance, if the stable tetragonal type was cooled slowly.