The effects of O-2 and H2O on the electrical characteristics of pentacene thin film transistors have been investigated using in situ and ex situ I-V measurements. We found that O-2 enhances the saturation current by about 10%, and the mobility is only slightly increased. The effect of H2O on the device performance is considerably complicated. The physically absorbed water degrades the device performance, and the chemically absorbed water interacts with pentacene to form pentacene/(H2O)(n) clusters, which enhance the saturation current. On the other hand, the effect of H2O is found to be partly reversible after repumping to vacuum due to the undissociated clusters, which attribute to the variations of the saturation current and field effect mobility. (C) 2004 Elsevier B.V. All rights reserved.