We have investigated the microstructure and properties of YBa2Cu3O9−&dgr;that contain resistive transitions at 90 and near 290 K using high‐resolution and analytical transmission electron microscopy and Raman scattering techniques. The specimens contain normal orthorhombic (Pmmmspace group) phase associated withTc=90 K, and another phase which we tentatively assign it to be associated with the resistive transitions near 290 K. The new phase has acaxis larger by about 16% and it grows epitaxially on the {100} planes of the orthorhombic (tripled perovskite unit cell) phase havingTc=90 K.