The enthalpies of dilution of micellar solutions of several 12-s-12 dimeric surfactants: of the alkanediyl-alpha,omega-bis(dodecyldimethylammonium bromide) type, differing by the carbon number s of the alkanediyl spacer, and of dodecyltrimethylammonium bromide (DTAB) have been measured calorimetrically, in a range of concentrations extending from well below to well above the critical micelle concentration (cmc). The results permitted the determination of the enthalpy of micellization, DeltaH(M)(o), of the investigated surfactants at 25 and 35degreesC. The values of DeltaH(M)(o) were always negative and became more negative as the temperature was increased. The plot of -DeltaH(M)(o) against s showed a shallow minimum at about s = 5 and a large decrease of -DeltaH(M)(o) going from 12-2-12 to 12-4-12. This effect has been attributed to the contribution to DeltaH(M)(o) of the hindered rotation of the dodecyl chains around the spacer C-C bond for 12-2-12. This hindrance is shown to rapidly disappear when s is increased from 2 to above 4. The specific heats of micellization, the free energies of micellization, and the entropies of micellization (DeltaS(M)(o)) have been calculated using the DeltaH(M)(o) values and the reported cmc and micelle ionization degree data for 12-s-12 surfactants: and DTAB. For all surfactants the results show that TDeltaS(M)(o) > -DeltaH(M)(o), indicating an entropy-driven micellization.