The equilibrium micellar structure in a tris(2-hydroxyethyl)(tallowalkyl)ammonium acetate (TTAA)/sodium salicylate (NaSal) surfactant system has been studied using light scattering and low-shear viscometry methods. Scattered Light intensity autocorrelation functions were measured and the hydrodynamic correlation lengths were obtained for solutions with surfactant concentrations ranging from 1 to 5 mM and NaSal/TTAA ratios from 0.6 to 5 at temperatures from 10 to 80 degrees C. From the temperature dependence of the hydrodynamic correlation length, the concentration, counterion/surfactant ratio, and temperature at which the free micelles begin to overlap were inferred. The overlapping surfactant concentration decreases with increasing counterion/surfactant ratio, and the overlapping ratio decreases with increasing surfactant concentration. The overlapping temperature increases with increasing surfactant concentration and counterion/surfactant ratio. "Zero shear rate" viscosities of these solutions at 25 degrees C were measured to estimate the overlapping concentrations, and the results are compared to light scattering results.