Activity changes of sodium xylenesulfonate (SXS) in lecithin/SXS/water microemulsion phase were determined by equilibrating it with the SXS/water solution through a semipermeable membrane, which retained the lecithin while the water and SXS were free to move. The activity of the SXS in the microemulsion phase was calculated from its equilibrium concentration measured in the SXS/water phase. The activity changes were interpreted by both the surface excess approach and Hall's thermodynamic description. The calculation of the adsorbed amount of SXS took the electrostatic exclusion effect into consideration. Combining the microemulsion with water leads to the formation of vesicles. A relation between the size of these vesicles and the composition of the microemulsion droplets is suggested by considering a depletion effect.