The interaction of three cationic poly {9,9-bis[N,N-(trimethylammonium)hexyl]fluorene-co-1,4-phenylene} polymers with average chain lengths of ∼6, 12, and 100 repeat units (PFP-NR36(I),12(Br),100(Br)) with both double and single stranded, short and long, DNA and DNA bases have been studied by steady state and time-resolved fluorescence techniques. Fluorescence of PFP-NR3 polymers is quenched with high efficiency by DNA (both double and single stranded) and DNA bases. The resulting quenching plots are sigmoidal and are not accurately described by using a Stern-Volmer quenching mechanism. Here, the quenching mechanism is well modeled in terms of an equilibrium in which a PFP-NR3/DNA aggregate complex is formed which brings polymer chains into close enough proximity to allow interchain excitation energy migration and quenching at aggregate or DNA base traps. Such an analysis gives equilibrium constants of 8.4 × 10(6) (±1.2 × 10(6)) M(-1) for short-dsDNA and 8.6 × 10(6) (±1.7 × 10(6)) M(-1) for short-ssDNA with PFP-NR36(I).