Structural properties of tetracene thin films grown by vacuum sublimation on a flexible Mylar© 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 (α and β thin film phases) have been identified. They differ for the dh 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.