We investigate the exciton dynamics in composite systems of conjugated polymers and fullerene molecules by photoluminescence ~PL! and femtosecond transient absorption experiments. In solid mixtures ~blends! we find a strong concentration-dependent quenching of the polymer PL. This is attributed to an efficient electron transfer ~ET! from the photoexcited conjugated polymer to the fullerene. The ET dynamics is directly monitored by measuring the transient stimulated emission of the conjugated polymer. The transfer rate depends linearly on the C60 concentration and ranges between (66 ps) 21 and (5 ps) 21 for concentrations from 0.5% to 5%. This dependence is in accordance with an exciton diffusion process occurring prior to the ET. The exciton diffusion length in the conjugated polymer is directly determined by measuring the PL quenching in welldefined heterostructures comprising a self-assembled fullerene monolayer and a thin spin-coated polymer layer of variable thickness. From these measurements we infer a value of 14 nm for the exciton diffusion length in ladder-type poly ~p-phenylene!. Our results are of direct relevance for further optimization of polymer photovoltaic devices. @S0163-1829~99!07923-0#