Bottom-contact organic field-effect transistors (OFETs) based on poly(3-hexylthiophene)-2,5-diyl were fabricated under different process conditions. The devices displayed drastic differences in their ambient-air stability. Whereas it took only about 10 min in air for the off current to increase by one order of magnitude in OFETs prepared with chloroform and hexamethyldisilazane, a 120 min exposure to air caused only a slight degradation of OFETs prepared using 1,2,4-trichlorobenzene, n-octadecyltrichlorosilane, and a heat treatment. The differences in the film surface morphology were analyzed and possible mechanisms for the enhanced stability are discussed. (c) 2006 American Institute of Physics.