We examine the electronic and vibrational dynamics of a model conjugated polymer using a particle-hole treatment for electronic excitation described in Ref. 1. We observe the transition from a delocalized free exciton state to a self-trapped exciton, and compare the characteristics of the process of localization to those predicted by existing theories. We find that the reaction path to self-trapping involves a well-defined intermediate state, complicating the process of cooling for the self-trapped exciton. We also find that high-energy excitons do not couple strongly to the lattice, and therefore do not self-trap. (C) 2000 American Institute of Physics. [S0021-9606(00)70511-3].