Structural properties of tetracene thin films grown by vacuum sublimation on a flexible Mylar((c)) substrate have been investigated by means of synchrotron X-ray diffraction. The films are polycrystalline and are made up of crystalline domains oriented with the (0 0 l) planes almost parallel to the substrate and completely misoriented around the surface normal. Two crystallographic phases (alpha and beta thin film phases) have been identified. They differ for the d(h k l) interplanar spacing, both larger than that of the bulk. As a comparison, results from tetracene films grown on SiO2 have been reported to investigate the different charge transport properties of films grown on Mylar and on SiO2 substrates. (c) 2006 Elsevier B.V. All rights reserved.