This paper describes micelle exchange kinetics of hydrophobic pyrene derivatives between SIDS micelles in the presence of moderate concentrations of sodium counterions ([Na+] < 150 mM). The kinetics were followed by fluorescence time-scan measurements in which the disappearance of excimer over time was monitored. The exchange rate constant k(obs) is highly sensitive to the counterion concentration and increases as a power law against [Na+] with an exponent of 4. from a value of almost 0 in the absence of salt to 10(-2) s(-1) for [Na+] =100 mM. The exchange rate is not very sensitive to the concentration of SDS micelles, except as the SIDS concentration affects the ionic strength, which indicates that the kinetics are dominated by a first-order process. This process is attributed to a fission-growth mechanism in which the fission rate is rate-limiting. Although fission can yield any size micelles, the use of hydrophobic probes restricts the observation to the events yielding two micelles large enough to bear the probe molecules. We propose that the barrier to fission is the creation of surface instabilities, which are enhanced in the proximity of the micelle-to-rod transition. Over the range of counterion concentrations investigated here, micelle fusion is inhibited by electrostatic repulsion.