The binding of proflavine (D) to single stranded poly(A) (P) was investigated at pH 7.0 and 25°C using T-jump, stopped-flow and spectrophotometric methods. Equilibrium measurements show that an external complex PD I and an internal complex PD II form upon reaction between P and D and that their concentrations depend on the polymer/dye concentration ratio (C P /C D ). For C P /C D <2.5, cooperative formation of stacks external to polymer strands prevails (PD I ). Equilibria and T-jump experiments, performed at I=0.1M and analyzed according to the Schwarz theory for cooperative binding, provide the values of site size (g=1), equilibrium constant for the nucleation step (K ∗ =(1.4±0.6)×10 3 M −1 ), equilibrium constant for the growth step (K=(1.2±0.6)×10 5 M −1 ), cooperativity parameter (q=85) and rate constants for the growth step (k r =1.2×10 7 M −1 s −1 , k d =1.1×10 2 s −1 ). Stopped-flow experiments, performed at low ionic strength (I=0.01M), indicate that aggregation of stacked poly(A) strands do occur provided that C P /C D <2.5.