Electron-nuclear double-resonance spectra of undoped polythiophene (PT) are reported in comparison with those of regioregular poly(3-alkylthiophene) (RR-P3AT). In PT, the maximum spin density reaches 0.06, corresponding to the polaron half-width of about 8 thiophene rings. This is contrasted with the half-width of about 10 thiophene rings in regioregular poly(3-hexylthiophene), RRP3HT, and poly(3-octylthiophene), RR-P3OT. The observed spin densities of polarons in these polymers are compatible with theoretical values obtained for model oligomers. The different polaron extensions in PT and RR-P3AT are discussed in relation to the difference of local structures such as the planarity of the polymer backbones. (c) 2007 Elsevier B.V. All rights reserved.