The association between a randomly pyrene labeled PAA polymer (PAAMePy55) and a PEO−PPO−PEO triblock copolymer (P123) in aqueous solutions of different NaCl concentrations and pHs has been studied by means of dynamic light scattering (DLS) and steady-state fluorescence spectroscopy at 40 °C. At acidic pH values, in the low P123/PAAMePy55 molar ratio regime (i.e., at low P123 concentrations), the relaxation time distributions retrieved from the DLS data analysis were monomodal and very similar to those obtained for the pure PAAMePy55 solution. The apparent hydrodynamic radius of PAAMePy55 at low pH is 18 nm. At higher molar ratios (i.e., at high P123 concentrations), still in the acidic pH regime, bimodal relaxation time distributions were obtained, where the fast relaxation mode is connected to the translational diffusion of free P123 micelles with a hydrodynamic radius obtained at infinite P123 dilution (RH,P123=0) of 10−11 nm. This value coincides perfectly with the hydrodynamic radius of the pure P123 micelles at 40 °C, which was found to be ca. 10 nm at all pH values. The second mode corresponds to a complex consisting of one PAAMePy55 polymer chain and about 42 P123 micelles and with a RH,P123=0 between 35 and 36 nm depending on pH. At pH 9, the mixed system also presented bimodal relaxation time distributions. At this high pH, the intermolecular association between PAAMePy55 and P123 is less strong than at acidic pH according to the steady-state fluorescence measurements. The fast mode is also in this case attributed to free P123 micelles whereas the second mode is related to the so-called “slow mode” commonly observed for polyelectrolyte solutions. In this system, it is related to the formation of multichain domains, that is, large domains formed by several PAAMePy55 chains that move in a common electrostatic field (i.e., a structure factor effect). The presence of P123 micelles does not lead to the total disruption of these domains. They may either contain entrapped P123 micelles or hydrophilic diblock impurities (originating from the P123 sample) that associate with the PAAMePy55 chains. ; http://dx.doi.org/10.1021/jp8093885