Microcalorimetric measurements have been made on the series of gemini surfactants [CMH2M+1(CH3)(2)N-(CH2)(S)N(CH3)(2)CMH2M+1] Br-2, designated CMCSCMBr2, where M and S indicate the numbers of carbons in the side chains and spacer respectively, for M = 12 and S = 3, 4, 6, 8, 10, 12. For comparison, parallel measurements have been made on the series of double-chain, singly charged surfactants dodecyldimethyl-alkylammoninm bromides, designated as C12CNBr With N = 1; 2, 4, 6, 8, 10, 12, where N is the number of carbons in the secondary alkyl chain. The choice of this second series was to compare each CMCSCMBr2 with its nearest equivalent monomer C12CS/2Br The values of the critical micelle concentrations (cmc) were found to be in good agreement with other measurements, showing a maximum at S = 4-6 for the geminis and a nonlinear variation with chain length for the C12CNBr series. For both series of surfactants, the enthalpies of micellization, DeltaH(mic), are all exothermic and show a marked minimum in magnitude at S = 4-6 or N = 4-6. The variation of DeltaH(mic) and DeltaS(mic) across both series shows that the balance between enthalpic and entropic contributions to the process of micellization changes substantially with S or N.