The present paper investigates the cationic−anionicsurfactant mixtures with a view to understand therole of polar head packing in bringing about a vesicle to micelletransition. Differences in behavior betweenaddition of sodium dodecylbenzenesulfonate (SDBS) and sodium dodecylsulfate (SDS) to cetyltrimethylammonium-p-toluenesulfonate (CTAPTS) solutions areinvestigated by NMR and molecular modelingtechniques. It is concluded that the complex formed between SDSand CTAPTS is more tightly bound thanthat between SDBS and CTAPTS, leading to precipitation rather thanvesicle formation in the latter case.Molecular modeling shows that the SDBS chain has a bend in themolecule, and this facilitates loosepacking of the complex neccessary for the formation ofvesicles.