Small-angle neutron scattering has been used to obtain detailed information on local structure, including micellar cross-sectional radii, areas per headgroup, and number of surfactant molecules per unit length for mixed micelles of cetyltrimethylammonium 2,6-dichlorobenzoate and cetyltrimethylammonium chloride in water as a function of surfactant and salt concentrations and the relative amounts of the two counterions (the counterion inventory) at the micellar surface. The differing effects of penetrating (2,6 -dichlorobenzoate) and nonpenetrating (chloride) counterions on micellar surface charge density and on the relative energies of surfactant monomers in cylindrical vs end-cap micellar environments are elucidated.