The electron spin resonance (ESR) properties of a polystyrene (PS) blend of iodine-doped regiorandom (RRa) poly(3-octylthiophene) (P3OT) are investigated to reveal the intrachain spin dynamics of conjugated polymers. The PS blends exhibits broader and anisotropic ESR spectra compared to the isotropic spectrum of the pure RRa-P3OT film. Spectral simulations are performed to reveal the ESR features of the PS blend. It is concluded that intrachain spin motion is less active than interchain motion. In addition, the spin dynamics of the pure film are dominated by intrachain spin motion at low temperature and interchain motion at high temperature. (C) 2010 Elsevier B.V. All rights reserved.