We have used microcalorimetry to measure the critical micelle concentrations (CMC) and enthalpies of micellization (DeltaH(mic)) of the series of dissymmetric cationic gemini surfactants, [CmH2m+1(CH3)(2)N(CH2)(6)N-(CH3)(2)CnH2n+1]Br-2, designated as CmC6CnBr2, with constant n + m = 24 and n = 6, 8, 10, 11, 12. There is a small decrease in CMC as the ratio m/n increases but this decrease is not much larger than the error. On the other hand there is a very large decrease in the enthalpy of micellization with m/n. This is consistent with a large increase in the hydrophobic contribution to micellization with m/n. To explain the relative invariance of the CMC. there must be a corresponding large increase in the entropy of micellization with m/n. The change in the hydrophobic contribution is explained in terms of the relative strengths of internal and external hydrophobic contacts down the series.