The paper reports on the microstructure of rutile TiO2 films grown on r-plane sapphire surfaces by molecular beam epitaxy as a function of oxygen flux during growth. Single-phase, epitaxial rutile films were obtained for all growth conditions. X-ray diffraction and transmission electron microscopy showed that the films contained a high density of twins, likely as a result of different variants nucleating on the substrate simultaneously. At low oxygen fluxes, surface features were dominated by twin variants. With increasing oxygen flux, the surface morphology changed due to the reduced mobility of ad-atoms. The relationship between the arrangements of oxygen octahedra in the sapphire and rutile structures, the epitaxial orientation and twin formation is discussed. (C) 2009 Elsevier B.V. All rights reserved.