The phase equilibria for the binary (composition vs temperature) catanionic surfactant system hexylammonium hexanoate-water and the isothermal (298 K) ternary system hexylammonium hexanoate-water-hexane are studied. The surfactant forms two isotropic solution phases with water at 298 K, one between 0 and 4 wt% of surfactant and the other, between 56 and 100 wt% of surfactant. The two single phases are connected by a two-phase region. On heating to 80 degrees C, no substantial change of solubility of the surfactant with water is observed. For the ternary system, at very high oil content and low contents of both water and surfactant, the single-solution-phase region is very narrow; this increases its width measured from the water axis with increased catanionic and water concentrations. However, the dilute aqueous solution region has limited capability to solubilize oil. The solution structure is studied by measuring components’ selfdiffusion coefficients by the NMR technique. Surfactant, water, and oil molecules are found to diffuse rapidly, and the diffusion data do not support the presence of micelle-like aggregated structures in solutions.