Grazing incidence synchrotron X-ray diffraction was used to investigate the structure of vacuum-sublimated tetracene thin films (6–50 nm thick) deposited on silicon dioxide. The films were found to be polycrystalline with crystallite texturing and size increasing with the deposition flux. This last parameter was found to have a great influence on the relative amounts of the polymorphs (thin film α and β phases) composing the films. These two different phases are characterised by different spacings of the (0 0 ℓ) planes, the spacing of the α phase being closer to that of the bulk. The thicknesses of the two phases in the film change as a function of the deposition flux. Finally, we discuss the role of the deposition flux on the charge-carrier mobility in tetracene films used as active layers in field-effect devices.