The dynamics of unimer exchange between aqueous micelles, formed by two amphiphilic block copolymers, i.e., poly(styrene-b-sodium methacrylate) and poly(tert-butylstyrene-b-sodium methacrylate), has been investigated by steady-state fluorescence spectroscopy. The kinetics are so slow at room temperature that no exchange could be detected over several hours, while at 60 °C the exchange rate constants could be estimated. These results corroborate our previous findings that the rate is slowed down by increasing the hydrophobicity of the core. In addition to the temperature, the exchange can also be tuned by the addition of either a cosolvent or a cosurfactant. The efficiency of these additives to speed up the exchange process is related to their water solubility and their compatibility with the hydrophobic core of the micelles. The most pronounced effect on the exchange process is observed when the water solubility is low and the mixing of the additive with the hydrophobic core is favorable.