We present femtosecond optical Kerr effect (OKE) studies of the third-order non-linear optical response of poly (distyrybenzene-co-triethylene glycol) (PDTG) under both resonance and off-resonance conditions. The excited-state enhancement of OKE signal was observed under resonance excitation. The temporal behavior of resonant OKE signal was found to be characterized by a fast component with time constant of about 250 fs and a slow component with lifetime of about 420 ps, which are attributed to de-localized pi-electron and intra-chain singlet exciton migration among the backbone of the polymer, respectively. The off-resonance OKE signal only was seen to consist of an ultrafast response due entirely to delocalized electrons contribution. (C) 2004 Elsevier B.V. All rights reserved.