The performance and charge transport characteristics of pentacene transistors with oxide gates such as SiO2 and Al2O3 have been recently shown to largely depend on the density of residual carriers due to electron acceptor defects at the oxide surface. The threshold field in particular is strongly connected to the density of these surface states. We demonstrate that by vapor deposition of self-assembled monolayers on the oxide gate prior to pentacene growth this charge transfer process is inhibited, the residual charge concentration is decreased by up to two orders of magnitude, and the field-effect mobility increases by up to a factor of four.