Poly(3-hexylthiophene-2,5-diyl), P3HexTh, was p-doped (or oxidized) by sulfonic acids such as methanesulfonic acid in the presence of O(2) On the other hand, weak acids such as acetic acid did not bring about such p-doping. In the case of regioregular head-to-tail type P3HexTh, molecular assembly took place prior to the p-doping. p-Doping of P3HexTh with sulfonic acid is discussed in relation to self-doping of sulfonated polythiophenes. (C) 2002 Elsevier Science B.V. All rights reserved.