Abstract Organic field-effect transistors (OFETs) based on regioregular poly(3-hexyl-thiophene) (P3HT) have been demonstrated with improved ambient-air stability. Substrates of untreated SiO 2, octadecyltrichlorosilane (OTS)-treated SiO 2, and SiO 2 coated with a thin layer of polymethylmethacrylate (PMMA) were compared in terms of OFET electric performance and air stability. We are able to show virtually no change in transistor on/off ratio over a 3-day period in ambient air in OFETs fabricated on SiO 2-PMMA substrates. We also studied devices based on OTS-treated PMMA interface. By analyzing the surface morphology, possible mechanisms of the enhanced air stability are discussed.