Counterion-mediated micellar growth for cetyltrimethylammonium 2,6-dichlorobenzoate alone and together with cetyltrimethylammonium chloride has been investigated using small-angle scattering techniques. Variations in counterion composition at constant ionic strength cause large changes in micelle size. The selectivity coefficient for adsorption of the aromatic counterion over chloride ions at CTA(+) surfaces has been determined. The effects of the two counterions on micellar surface potentials, together with H-1 NMR chemical shift data, allow inferences to be drawn about the penetration of the aromatic counterion into the head group region of the micelles. The aromatic ion increases the surfactant-packing parameter by both increasing the average volume per surfactant monomer, a cosurfactant-like effect, and decreasing the area per head group by screening electrostatic repulsions. Counterions such as chloride, which show only surface adsorption, affect only area and are much less effective at driving micellar growth. Some comparisons are also made with literature data for CTABr.