Structure and morphology of picene films under vacuum and O-2 atmosphere were studied by in situ synchrotron X-ray scattering. We observed that picene films exhibit a highly oriented and ordered structure, which is similar to the one reported for picene single crystals. Furthermore, we found that the film structure determined under vacuum remains nearly unchanged under O-2 atmosphere. The results provide new insights into a high hole mobility and O-2 gas sensing mechanism previously reported for picene thin film-based organic field-effect transistors. (C) 2012 Elsevier B.V. All rights reserved.