Relative partial molar heat content curves were obtained for a nonionic surfactant, dodecyldimethylphosphine oxide, in H2O and D2O solutions from 15 to 40 degrees C by titration calorimetry. The critical micelle concentration (cmc) was always lower in D2O than in H2O. The enthalpy change for micelle formation was determined at 25 degrees C from integration of an abbreviated form of the van＇t Hoff equation assuming a temperature-independent aggregation number and heat capacity change to be 1.13 +/- 0.14 and 1.75 +/- 0.14 kcal/mol in H2O and D2O, respectively. The corresponding calorimetric values were 1.66 +/- 0.03 and 2.07 +/- 0.02 kcal/mol. The change in heat capacity obtained from the van＇t Hoff equation was -113 +/- 17 cal/mol-K in H2O and -140 +/- 11 cal/mol-K in D2O. The corresponding values determined from the temperature dependence of the molar enthalpy were -161 +/- 2 cal/mol-K in H2O and -171 +/- 2 cal/mol-K in D2O. The temperature dependence of the cmc was fairly well described in both solvents using the partial molar enthalpy and heat capacity changes that accompany micelle formation.