By applying various surface treatments to the oxide gate of pentacene transistors, both in vacuum (H2, O2 and Ar plasma treatments) and in aqueous solution, we were able to vary the balance between oxygen-free radicals (Si•), oxygen-centered radicals (Si−O• and Si−O−O•), and other defect types such as silanol (Si−OH), silicon hydride (Si−H), and silicate groups (Si−O−). The consequences of these modifications on the microstructure of pentacene deposited onto the gate have been studied by contact angle measurements and atomic force microscopy. The changes in the transport properties of the ultrathin devices were deduced from their electrical characteristics and from independent four-probe measurements. While the wetting properties of the oxide and in turn the pentacene growth process is mainly controlled by the Si−OH/Si−O− balance, it has been evidenced that oxygen-centered radicals act as charge transfer centers and determine to a large extent the transport properties of many thin-film transistors.