Dilute solutions of catanionic vesicles formed by the mixed single-chained (sodium dodecylsulphate, SDS) and double-chained (didodecyldimethylammonium bromide, DDAB) surfactants have been investigated by differential scanning calorimetry. It is for the first time reported a gel-to-liquid crystal phase transition temperature, T-m, in this type of mixed vesicles. The SDS-rich vesicles (at X-SDS = 0.71) show a concentration-dependent T-m in the range 9-16 degreesC. Addition of salt is seen to have an effect on T-m similar to that observed with increasing surfactant concentration, both inducing a decrease in T-m. These results differ from those obtained for neat DDAB vesicles. The observed effects in the two types of vesicles are rationalised in terms of headgroup electrostatic interactions which may have influence on the chain packing and phase transition temperature.