Abstract Thermal stability of the physical properties in oxygen-deficient YBa 2Cu 3O x has been investigated. Measurements of dc resistivity showed that metal to semiconductor transition takes place during annealing at temperatures up to 300 °C. Diamagnetic magnetization remarkably decreased with an increase in annealing temperature. This indicates that the volume fraction of superconducting phase diminishes with temperature. The electrical transition accompanies a structural change to tetragonal-like phase which is thought to have the semiconducting nature.