Synergistic　interactions　between　zwitterionic　surface　active　ionic　liquids　(SAILs),　N-alkyl-N　＇-carboxymethyl　imidazoliumm　inner　salts　([N-C-　n　,　N　＇-CO2-Im],　n=12,　14),　and　a　traditional　twin-tail　anionic　surfactant　(dioctyl　sulphosuccinate　sodium　salt,　AOT)　in　aqueous　solution　has　been　studied　by　surface　tension　and　fluorescence　measurements　at　298.15K.　The　results　imply　that　the　mixed　systems　have　superior　surface　activity　in　comparison　to　individual　surfactant,　and　they　all　present　a　non-ideal　behavior　with　synergism.　The　strength　of　interactions　between　two　surfactants　increases　with　the　enhancement　of　the　hydrocarbon　chain　of　the　zwitterionic　SAILs.　Compared　with　interaction　parameters　in　mixed　monolayer,　the　larger　value　of　interaction　parameters　in　mixed　micelles　indicates　that　the　mixed　micelle　formation　is　more　favorable.　Additionally,　a　series　of　thermodynamic　parameters　such　as　the　standard　Gibbs　free　energy　of　adsorption　(　[GRAPHICS]　),　the　standard　Gibbs　free　energy　of　micellization　(　[GRAPHICS]　),　and　excess　free　energy　of　micellization　(Delta　G　(ex))　have　been　calculated.　The　negative　values　of　[GRAPHICS]　and　[GRAPHICS]　show　that　both　micellization　and　adsorption　of　surfactants　are　spontaneous,　while　the　negative　value　of　Delta　G　(ex)　indicates　the　formation　of　stable　mixed　micelles　for　[N-C-　n　,　N　＇-CO2-Im]/AOT　mixture,　which　can　potentially　be　useful　for　industrial　preparations　and　medicinal　formulations.　[GRAPHICS]　/＞