The aggregation behavior of sodium 3,6,9,12,15-pentaoxaheptacosanoate (AEC4-Na) in aqueous solution with increase of the concentration at 25 degrees C was investigated using differential scanning calorimetry,, equilibrium surface tension, solubilization of an oil soluble dye, steady-state fluorescence, dynamic light scattering, and freeze fractured transmission electron microscopy. Vesicle formation of AEC4-Na preceded micelle formation below the critical micelle concentration (cmc). The vesicle-to-micelle transition was observed through a narrow concentration region above the cmc. The mean diameters of the vesicles and micelles were not affected by the concentration. All solutions over a wide range of concentrations were homogeneously transparent with a low Krafft point below 0 degrees C. These results indicate that the AEC4-Na vesicles have a thermodynamically stable structure. Vesicle formation may be caused by a pseudobinary mixed surfactant system composed of monomeric AEC4-Na and an acid soap that consists of a dimer complex formed between AEC4-Na and unneutralized AEC4-Na. The thermodynamic stability would then result from the inhibition of close intermolecular aggregation and flexibility of the molecular shape in the vesicles due to the oxyethylene units in AEC4-Na.