The changes in enthalpy, entropy, and heat capacity that accompany micelle formation for sodium, octyl, decyl, and dodecyl sulfate, dimethyldecyl and dimethyldodecyl phosphine oxide, sodium dodecoyl sarcosinate, and dodecylpyridinium chloride and bromide were determined by thermometric titrations between 21 and 35° in aqueous solution. The enthalpy and entropy changes exhibited a correlation for a given chain length consistent with a compensation temperature of about 300°. In general, the enthalpy and entropy changes, which appear to be rather complex parameters, did not parallel the heat capacity change. The change in heat capacity for micellization appears mainly to reflect solvent effects. Sodium dodecyl sulfate was also studied in 6 M urea, 10% ethanol, 0.1 M sucrose, 0.1 M NaC1, 0.5 M NaCl, and 0.1 M NaBr. A comparison of enthalpy changes that have been determined by calorimetry and from the temperature dependence of the CMC indicates that the two approaches should not be assumed to yield identical results.