Microcalorimetric measurements have been made as a function of temperature on the series of gemini surfactants [CMH2M+1(CH3)(2)N(CH2)(S)N(CH3)(2)CMH2M+1]Br-2, designated CMCSCM, where M and S indicate the numbers of carbon atoms in the side chains and spacer, respectively, for M = 12 and S = 3, 4, 6, 8, 10, and 12. The maximum at S = 4-6 in the values of the critical micelle concentrations (cmc's) as a function of spacer length was found to be associated with a strong minimum in the magnitude of the exothermic enthalpy of micellization, DeltaH(mic). A strong maximum in the corresponding entropies of micellization, DeltaS(mic), almost cancels out the effect of DeltaH(mic), so that DeltaG(mic) and hence the values of the cmc show a much weaker variation with spacer. The values of Delta Cp pass through a marked minimum and are large, corresponding to a large variation of DeltaH(mic) with temperature. This variation of DeltaH(mic) has some interesting consequences for the variation of the cmc. Thus, for C12C12C12 the contribution of DeltaH(mic) to DeltaG(mic) at room temperature is about 36%, but this increases through 61% to 84% at 308 K. Over this range the changes in DeltaG(mic) remain within about 10% of each other, almost within the experimental error.