In this paper we present a detailed characterization of metal-isolator semiconductor MIS structures and organic thin film transistors (OTFTs) using poly(methyl methacrylate) (PMMA) as gate dielectric on top of a semiconductor poly(3-hexylthiophene) (P3HT) layer. The PMMA layer was spin coated from a 6% dilution of PMMA in anisole. The P3HT layer was spin coated from a 0.66 wt% dilution of P3HT in chloroform. OTFTs with upper gate were fabricated using photolithographic processes. The current density across the dielectric is below 1 x 10(-6) A/cm(2). The interface states density is below 1 x 10(11) cm(-2), while the flat band voltage shift is less than 0.5 V for bias stress in the range of 20 V. Accumulation occurs for gate voltage below -10 V, allowing OTFTs to work in the voltage range below -30 V, with threshold voltage around -2.5 V. Mobility was 2.5 x 10(-3) cm(2)/V s, which is among highest values reported for P3HT OTFTs working in this voltage range. (C) 2007 Elsevier Ltd. All rights reserved.