Bimolecular　interactions　between　excitations　in　conjugated　polymer　thin　films　are　important　because　they　influence　the　efficiency　of　many　optoelectronic　devices　that　require　high　excitation　densities.　Using　time　-　resolved　optical　spectroscopy,　we　measure　the　bimolecular　interactions　of　charges,　singlet　excitons,　and　triplet　excitons　in　intimately　mixed　polyfluorene　blends　with　band-edge　offsets　optimized　for　photoinduced　electron　transfer.　Bimolecular　charge　recombination　and　triplet　-　triplet　annihilation　are　negligible,　but　exciton-charge　interactions　are　efficient.　The　annihilation　of　singlet　excitons　by　charges　occurs　on　picosecond　time-scales　and　reaches　a　rate　equivalent　to　that　of　charge　transfer.　Triplet　exciton　annihilation　by　charges　occurs　on　nanosecond　time-scales.　The　surprising　absence　of　nongeminate　charge　recombination　is　shown　to　be　due　to　the　limited　mobility　of　charge　carriers　at　the　heterojunction.　Therefore,　extremely　high　densities　of　charge　pairs　can　be　maintained　in　the　blend.　The　absence　of　triplet　-　triplet　annihilation　is　a　consequence　of　restricted　triplet　diffusion　in　the　blend　morphology.　We　suggest　that　the　rate　and　nature　of　bimolecular　interactions　are　determined　by　the　stochastic　excitation　distribution　in　the　polymer　blend　and　the　limited　connectivity　between　the　polymer　domains.　A　model　based　on　these　assumptions　quantitatively　explains　the　effects.　Our　findings　provide　a　comprehensive　framework　for　understanding　bimolecular　recombination　and　annihilation　processes　in　nanostructured　materials.　©　2010　American　Chemical　Society.