gemini surfactants in aqueous solutions and to explore the origin of their very low CMCs and their unusual ability to generate large micelles when the spacer length is small. For this purpose, we develop expressions for various contributions to the free energy of micellization of gemini surfactants by modifying our previous treatment of single chain ionic surfactants [16] and [17]. The modifications account for important spacer-related effects, namely, the presence of a part of the spacer at the micelle core–water interface, the non-uniform distribution of ionic head groups at the micelle surface, the packing constraint on the tails due to their connectivity via the spacer, and the possible burying of a part of the spacer inside the micelle core.