The decay dynamics of a partially conjugated polymer has been studied by femtosecond fluorescence upconversion spectroscopy. Dilute polymer solutions exhibit wavelength independent decay, indicating the existence of a well-defined chromophore in the polymer chains. Concentrated polymer solutions or solutions with poor and good solvent exhibit an increased decay time as the emission wavelength increases, attributed to aggregates. Energy transfer from isolated to aggregated chains is hindered and this is due to exciton localization. The transient fluorescence spectra of the polymer in poor-good solvent exhibit a red shift as well as a broadening, both attributed to the aggregates emission. (C) 2004 Elsevier B.V. All rights reserved.