In this work we present an X-ray diffraction study of the early stages of growth of an organic semiconductor (sexithiophene, T6) thin film prepared by high vacuum sublimation. Specular reflectometry and grazing incidence X-ray diffraction were used to monitor the formation of T6 films on silicon oxide. Our results show that T6 grows as a crystalline layer from the beginning of the evaporation. The reflectometry analysis suggests that, in the range of rates and temperatures studied, the growth is never layer by layer but rather 3D in nature. In-plane GIXD has allowed us to observe for the first time a thin film phase of T6 formed of molecules standing normal to the substrate and arranged in a compressed unit cell with respect to the bulk, i.e. the unit cell parameters b and c are relatively smaller. We have followed the dynamics of formation of this new phase and identified the threshold of appearance of the bulk phase, which occurs above ≈5–6 monolayers. These results are relevant to the problem of organic thin film transistors, for which we have previously demonstrated experimentally that only the first two monolayers of T6 films are involved in the electrical transport. The layers above the second one do not effectively contribute to charge mobility, either because they are more “disordered” or because of a screening of the gate field.