The interaction in two mixtures of an anionic gemini surfactant having N,N-dialkylamide and carboxylate groups in a molecule, (CH2)(2)[N(COC11H23)CH(CO2H)CH2(CO2H)](2).2NaOH (GA), and conventional anionic surfactants have been investigated in 0.1 M NaCI at pH 5.0. The two mixtures are GA/sodium dodecylsulfate (SDS) and GA/sodium N-dodecanoylglutamate (AGS) at a molar fraction of GA, alpha(GA) = 0.25, Mixtures of both GA/SDS and GA/AGS exhibit synergism in surface tension reduction effectiveness. The GA/SDS mixture also exhibits synergism in surface tension reduction efficiency and mixed micelle formation, whereas the GA/AGS mixture does not. The interaction in mixed adsorption film formation is stronger than that in mixed micelle formation for the two mixtures. The interaction in the formation of the mixed adsorption film and the mixed micelle for the GA/SDS mixture is stronger in both formations than that for the GA/AGS mixture. The stronger interaction for the GA/SDS mixture may be caused by the combination of the smaller minimum area per molecule at the air/water interface (Amin) of the head groups in the GA molecule and the larger Amin in the SDS molecule. (C) 2003 Elsevier Science (USA). All rights reserved.