Mixed alkyl/fluoroalkyl phosphonic acid self-assembled monolayers have been prepared as ultra-thin dielectrics in low-voltage organic thin-film transistors and complementary circuits. Mixed monolayers enable continuous threshold-voltage tuning simply by adjusting the molecular mixing ratio Continuous threshold-voltage control makes it possible to place the switching voltage of the circuits at precisely half the supply voltage, producing the maximum noise margin. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.