By using absorption and fluorescence (steady-state and time-resolved) techniques, the interaction between a poly(acrylic acid) (PAA), randomly grafted with pyrene (Py) units (PAAMePy55), and a triblock copolymer of poly(ethylene oxide) and poly(propylene oxide) (EO20PO68EO20, P123) was investigated. From the fluorescence data, it is shown that upon addition of P123 a decrease of the (pyrene−pyrene, Py−Py) intramolecular association, i.e., a decrease of dynamic and static excimer formation, is observed. Time-resolved fluorescence data reveal the existence of two types of monomers (monomers that are able to form excimer, MAGRE, and isolated monomers) and two excimers. Addition of P123 causes also an increase of the amount of isolated Py monomers. The overall fluorescence data suggest that the PAAMePy55 and the P123 block copolymer associate strongly at low pH, leading to the formation of P123 micelles surrounded by one PAAMePy55 chain, where the pyrene groups are located at the PPO/PEO interface of the P123 micelles. Steady-state fluorescence results also showed that an excess of P123 micelles in solution is required for the association to occur. At high pH (pH 9 and above) the situation is less clear. The steady-state (particularly in the I1/I3 ratio) and time-resolved fluorescence results indicate a contact between the pyrene groups and PEO, which then would imply that there may be an interaction, but much weaker than at low pH. ; http://dx.doi.org/10.1021/jp8093879