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The electrical characteristics of pentacene organic thin-film transistors (OTFTs) using cross-linked poly(methyl methacrylate) (PMMA) as the gate dielectric are reported. Ultra-thin films of cross-linked PMMA could be obtained by spin-coating and subsequent irradiation using a 1.515MeV ⁴He⁺ ion beam. The resulting film, with a thickness of 33nm, possessed a low leakage current density of about 10⁻⁶Acm⁻² for fields up to 2MVcm⁻¹. OTFTs incorporating the cross-linked dielectric operated at relatively low voltages, <10V, and exhibited a mobility of 1.1cm²V⁻¹s⁻¹, a threshold voltage of −1V, a sub-threshold slope of 220mV per decade and an on/off current ratio of 1.0×10⁶.