Oxygen-deficient, tetragonal thin films of YBa2Cu3O6+x with x ≈ 0.25, quenched from the deposition temperature, change to the oxygenated, orthorhombic phase with x ≈ 1, between 200 °C and 400 °C in flowing oxygen. The transition is not reversible in flowing oxygen, and cannot be completely reversed by cooling in flowing argon. We do not observe a transition of the orthorhombic films to the tetragonal phase up to 800 °C in flowing oxygen. We observe that the major impurity phases to appear under nonoptimal annealing conditions are oriented phases of YcuO2 and BaCu2O2, with Y2BaCu2O5 and Y2Cu2O5 conspicuously absent. These conclusions have been drawn from a study that uses perturbed angular correlation spectroscopy to probe the local microstructure of the films.