Samples of a high-temperature superconductor YBa2Cu3O7-x (orthorhombic phase) showed no significant weight loss in nitrogen up to 1173 K; however, differential thermal analysis measurements show that restructuring/decomposition begins around 1121 K. No reaction with alumina was found after prolonged heating at 1073 K. Electrical properties between 100 Hz and 1 MHz were generally stable at high temperatures, with little variation in properties at 1 MHz in inert and oxidizing atmospheres. Surface oxygen can be removed at high temperatures in flowing argon causing erratic electrical behaviour at lower frequencies and lower temperatures, which can be associated with changes in the oxygen conte nt, x, and partial quenching to the high-temperature tetragonal phase. Stability and electrical tests after pretreatment of YBCO-alumina composites at 933 K in CO2 or steam showed partial decomposition to BaCO3, CuO and Y2Cu2O5 and a phase transition from orthorhombic to tetragonal in the YBa2Cu3O7-x. The original state could be retrieved by calcination in air at 1073 K.