We have determined the effect of elevated temperature on the crystal structure and lattice parameters of the single‐phase high‐temperature superconductor YBa2Cu3O7using high‐temperature x‐ray powder diffractometry. This compound is orthorhombic below about 550 °C (depending on oxygen partial pressure) and transforms to a tetragonal polymorph at higher temperatures. The reversibility of this transition depends on the oxygen partial pressure. An oxygen ‘‘hopping’’ mechanism with possible oxygen loss, which appears consistent with the known structural details, is proposed for the transition.