The surface tension of an aqueous solution of decyltrimethylammonium decyl sulfate (DeTADeS) was measured as a function of temperature T at various molalities (m) over cap under atmospheric pressure. DeTADeS has been found to form equilibrium multilamellar vesicles (MLV) spontaneously. The surface density, the entropies of adsorption, and the entropy of vesicle formation are evaluated. The mechanism of formation of equilibrium vesicles is investigated from the standpoint of thermodynamics and from the comparison of the results with those of the micelle-forming systems. From the relatively small change of the surface density (Gamma)over-cap(H) on T at a given (m) over cap, the adsorbed film is implied to be tightly packed due to the strong electrostatic attraction between the polar headgroups. The energy change associated with adsorption from the vesicular state per mole of surfactant Delta U-H(V) is positive in the entire temperature range; thus, the curved bilayer in MLV is energetically more favorable than the planar adsorbed him. From the negative values of the entropy of vesicle formation Delta S-V(W), it is concluded that vesicle formation is driven by enthalpy whereas micelle formation is mostly entropy driven.