The generation of excitons and their interaction with holes in films of neat regioregular poly(3-hexylthiophene) and the polymer blended with 1 wt% of the electron-acceptor 6,6-phenyl-C61-butyric acid methyl ester (PCBM) have been studied using flash-photolysis time-resolved microwave conductivity. The sublinear relationship between the photogenerated hole density and the incident light intensity, in both the neat polymer and the donor−acceptor blend, can be attributed to the quenching of excitons by holes, at a rate characterized by a second-order rate constant (γ2) of 3 × 10−8 cm3/s. This value is larger than that found for other, luminescent conjugated polymers; the difference may be attributed to a greater collision probability, due to the higher mobility of the interacting species, or to an enhancement of the quenching rate once they are in close proximity. The phenomenon has consequences for the ultimate efficiency of organic photovoltaic solar cells that are based on the simple polymer:PCBM bulk heterojunction, especially under conditions of solar concentration.