Abstract The 90° rotation boundaries that occur widely in YBa 2Cu 3O 7- x and related oxide superconductors, and which are responsible for the c-⊥ to c-‖ transition in thin films, have been studied by high-resolution transmission electron microscopy. Comparison of image simulations to experimental micrographs allows selection between several models for the atomic structure at the interface. Interfacial misfit discolations with a Burgers vector of 1 3  have been observed. Many of these partial discolations move to stand-off positions in the interfacial grain with c aligned along the boundary, leaving behind a connecting stacking fault. The detachment of the discolations from the interface appears to be driven by chemical non-stoichiometry rather than by mechanical image forces. The epitaxial alignment of Y 2O 3 particles within the film and the presence of a stacking fault connecting one such Y 2O 3 inclusion with a 90° rotation boundary are also discussed.