This paper describes micelle exchange kinetics as a function of temperature for a pyrene derivative 1, a triglyceride containing a pyrenebutyrate ester, solubilized in aqueous micelles of Triton X-100. The kinetics were followed by stopped-flow fluorescence time-scan measurements in which the disappearance of excimer over time was monitored. Two components of the exchange process could be separated: a first-order process with an activation energy of 110 kJ/mol, and a second process with an activation energy of 160 kJ/mol. Because the second-order process had a rate (k(2) approximate to 1 x 10(6) M-1 s(-1)) at 24.6 degrees C that was nearly independent of the pyrene probe (1, 1-octylpyrene, 1-dodecylpyrene), the second-order process was assigned to a fusion-fragmentation mechanism, rather than a "sticky-collision" mechanism. The rates of the first-order process (k(1) approximate to 12 s(-1) for 1) at 24.6 degrees C increased with decreasing size of the probe. This step was attributed to a fragmentation-growth mechanism in which the fragmentation rate is rate limiting. Exchange by this mechanism must involve fragmentation of a micelle containing two probe molecules to form two sub-micelles large enough to bear one of the probes.