Polythiophenes (PTs), particularly regioregular poly(3-hexylthiophene-2,5- diyl) (rr-P3HT), are important materials in photovoltaics. The photophysical properties of PTs are still poorly understood, because of their aggregation tendency and formation of interchain species which can be avoided by insulating the conjugated backbone via self-threading. We investigated two polymers, rr-P3HT and its insulated analog, imbedded in PMMA at low concentrations. The exciton decay dynamics and fluorescence quantum yield were analyzed as a function of excitation power densities over the range from 1 × 10 -4 to 100 W/cm2. For both polymers, substantial (up to 5 times) photoinduced fluorescence quenching was observed owing to singlet-triplet annihilation and quenching by other long-living charged photoproducts. We found that chain insulation eliminates static (or ultrafast) fluorescence quenching, but has no effect on slow dynamic quenching at time scales longer than 10 ps. We propose that static quenching is solely due to chain aggregation, whereas the dynamic quenching is a consequence of intrachain processes.