Among the various materials suitable for organic thin-film transistors (OTFTs), pentacene stands out as a model molecule, exhibiting one of the highest field effect mobilities reported so far. Understanding the growth mechanism of pentacene on dielectrics is essential for controlling film morphology and for fabricating high quality, large-grain, defectfree films. Such films will help us gain insight into the fundamentals of transport in organic films as well as enable the ultimate OTFT performance. In situ synchrotron X-ray scattering was used to probe the early stages of pentacene growth on SiO 2 in real time and under conditions relevant to the fabrication of OTFTs. Reflectivity measurements reveal that a thin layer of water, initially present on the substrate, is trapped at the interface when the pentacene film is deposited. Therefore, the interface relevant for OTFTs is that between pentacene and water. Anti-Bragg oscillations, observed for the first time during organic film growth, reveal that the first monolayer of pentacene completes fully before the second one nucleates. Subsequent layers nucleate before the underlying layers complete, giving rise to the characteristic island structure observed in pentacene films used in OTFTs. A simple distributed growth model was found to adequately describe the growth mechanism.