We present nanoscale spatial extent of photogenerated polarons in a conjugated polymer, regioregular poly(3-octylthiophene). Light-induced electron-nuclear double-resonance (LENDOR) signals are clearly observed for the excitation around 1.8 eV due to the highly efficient photoinduced electron transfer in the polymer/fullerene composite. Broad LENDOR shifts directly reflect the spin density distribution of the polarons in the polymer. We determine its maximum shift using LENDOR-induced electron spin resonance, and evaluate the spatial extent of the polarons as similar to10 thiophene rings (similar to4 nm). (C) 2003 Elsevier B.V. All rights reserved.