We have investigated the photoluminescence (PL) characteristics of poly(3-octylthiophene) (P3OT) diluted in a solid matrix of polypropylene (PP). Using time-resolved PL spectroscopy, we have identified a continuous increase in PL quantum efficiency and in PL lifetime with decreasing concentration of P3OT in PP. The concentration-dependence of its decay profile has been analyzed according to the change in the PL efficiency. The analysis indicates that there is no large difference in the branching ratio to singlet emissive excitons between P3OTs in the spin-coated film and in the diluted sample, which leads to the conclusion that its radiative rate increases and its non-radiative rate decreases by the dilution in PP. (C) 2005 Elsevier B.V. All rights reserved.