The interaction between homologous surfactants in mixed micelles was studied by the Regular Solution Theory of mixed micelles. The interaction is independent of the nature of the polar head groups and attractive and the interaction parameter M depends linearly on the difference in chain length Delta n(C). The interaction becomes ideal at Delta n(C) = 0.75 +/- 0.06. Above Delta n(C) approximate to 5, the dependence remains linear but the slope increased 2.7 times. The phenomenon is explained as the effect of the reduction of the hydrocarbon/water micelle interface and a better packing of the chains in the micelle core, caused by the inclusion of a shorter homologous surfactants. This reduction can be more effective when Delta n(C) >= 5. (c) 2006 Elsevier Inc. All rights reserved.