The present paper investigates the cationic-anionic surfactant mixtures with a view to understand the role of polar head packing in bringing about a vesicle to micelle transition. Differences in behavior between addition of sodium dodecylbenzenesulfonate (SDBS) and sodium dodecyl sulfate (SDS) to cetyltrimethyl-ammonium-p-toluenesulfonate (CTAPTS) solutions are investigated by NMR and molecular modeling techniques. It is concluded that the complex formed between SDS and CTAPTS is more tightly bound than that between SDBS and CTAPTS, leading to precipitation rather than vesicle formation in the latter case. Molecular modeling shows that the SDBS chain has a bend in the molecule, and this facilitates loose packing of the complex neccessary for the formation of vesicles.