The dynamics of the exchange of chains between micelles formed by diblock copolymers in dilute solution in a selective solvent has been studied using fluorescence measurements. The samples are polystyrene-block-poly(oxyethylene) with a single fluorescent label (either naphthalene or pyrene) covalently attached at the junction between the blocks. The critical micelle concentration (cmc) of each sample can be determined from the concentration dependence of the integrated emission intensity, provided the cmc is high enough to permit detection. In order to study the kinetics, micelles of the two differently labeled samples were first prepared at the same concentration, solvent, and temperature, but in two different containers. The contents of the containers were then mixed, and the efficiency of nonradiative singlet energy transfer from naphthalene to pyrene was measured as a function of time. The time dependence of the intensity of the emission from naphthalene can be fitted to a sum of two exponentials, with time constants that differ by at least an order of magnitude. Activation energies are somewhat smaller for the faster process than for the slower process, but in both cases they are on the order of 10(2) kJ mol(-1) under the conditions where they can be measured. We have not been able to account for this result with a kinetic scheme that assumes the exchange of chains between the micelles takes place exclusively via the population of free chains. This difficulty suggests that an additional mechanism for the exchange of chains may be active.