The aggregation numbers of sodium dodecyl sulfate (SDS) micelles and the fluorescence quenching constants, k(q), of pyrene by CU2+ have been measured as a function of SDS concentration from 25 to 200 mM. The results confirm an empirical observation made by small angle neutron scattering (Bezzobotnov, et al. J. Phys. Chem, 1988, 92, 5738) that SDS micelles grow linearly as the one-fourth power of the total detergent concentration. The aggregation numbers, computed under the assumption that the CU2+ ions are distributed randomly among the micelles show a positive slope with increasing CU2+ concentration, contrary to theoretical expectations; however, invoking, a small electrostatic repulsion between CU2+ ions residing upon the same micelle brings the experimental observations into agreement with theory. This electrostatic repulsion decreases from epsilon(2) = 0.11 to 0.04kT (T = absolute temperature, k = Boltzmann constant) as the micelle size increases from 50 to 85 molecules. These electrostatic repulsion energies are comparable to those necessary to bring electron paramagnetic resonance results into agreement with theory as presented in the following paper. The quenching constant decreases with the size of the micelle according to k(q) = gamma D/R(2) with a coefficient of correlation r = 0.994, consistent with a model of diffusion encounters between reactants moving on or near a sphere of radius R. D is the relative diffusion coefficient of the reactants and gamma is a constant. Taking R to be the radius of the micelle and reasonable estimates of gamma shows that the diffusion coefficient of CU2+ is Of the same order of magnitude as it is in water. This leads to the conclusion that the quenching rate constant is nearly diffusion controlled and that pyrene is readily available at the micelle surface to participate in molecular collisions with CU2+. Further, the residence time of the CU2+ upon any given head group must be rather short. Even though the data are better fit under a hypothesis of electrostatic repulsion, the conclusions that the micelles grow as the one-fourth power of the detergent concentration and that the quenching is consistent with a surface diffusion mechanism are unaffected if a random distribution is utilized. Under this latter interpretation, the micelles grow from 57 to 89 molecules as the SDS concentration increases from 25 to 200 mM. The observed micelle growth together with an absence of substantial polydispersity is inconsistent with thermodynamic predictions.