The performance of thin-film transistors based on an aluminum oxide gate dielectric and a pentacene semiconductor have been systematically enhanced by modification of the oxide surface. The oxide layer was formed at room temperature using a standard anodization technique. The effects of surface modification of the oxide layer on device performance have been investigated. Higher mobility transistors were fabricated by passivating the oxide layer with a self-assembled molecular monolayer of an alkyl phosphonic acid or a spin-coated acrylic film. In both cases the normalized sub-threshold swing was reduced by a factor of 1.9 and 1.5, and the maximum charge-carrier mobility was increased by one and two orders of magnitudes respectively. The much increased mobility for transistors modified by a thin film of polymethylmethacrylate (PMMA) is attributed to the larger pentacene grain size.