The interaction enthalpies between didodecyldimethylammonium bromide (DDAB) and sodium dodecyl sulfate (SDS) have been measured at 298.15 K using an isothermal titration microcalorimeter. From the variation of enthalpy with SDS concentration, the enthalpy of interaction between DDAB micelles and SDS, the enthalpy of formation of the DDA(+)SD(-)(cryst), the enthalpy of vesicle formation of DDAB -SDS, and the enthalpy of transition from-vesicles to SDS-rich micelles have been determined to be -29.53, -125.8, 41.23, and 32.10 kJ/mol, respectively. The phase behavior of the DDAB-SDS aqueous mixture has also been investigated by turbidity measurements and transmission electron microscopy.