The temperature and electric field dependence of the hole mobility in regioregular poly(3-hexylthiophene) (P3HT) is studied by time-of-flight (TOF) technique. It is observed that above similar to250 K the slope of the electric field dependence of mobility becomes negative. Such phenomenon has been reported previously for a variety of amorphous charge transport materials, but not clearly observed in conjugated semiconducting polymers. The temperature and electric field dependence of mobility is analyzed both in the framework of Gill's phenomenological data analysis and disorder formalism. The occurrence of negative field dependence in the measured regioregular P3HT samples is attributed to the small energetic (sigma = 70 meV), but large spatial disorder (Sigma = 3.4). (C) 2004 Elsevier B.V. All rights reserved.