The relative partial molar enthalpy of decyldimethylphosphine oxide has been determined in H2O and D2O from infinite dilution to about 8 mm at 25 degrees C, The shapes of the resulting curves resemble those previously reported for ionic but not nonionic surfactants and are inconsistent with existing models of micelle formation, The enthalpy of micelle formation (corresponding to the maximum value of the relative partial molar enthalpy of the surfactant) was 12,900 +/- 200 cal/mol in H2O and 10,600 +/- 200 cal/mol in D2O, The cmc was lower in D2O than in H2O as expected (3.64 versus 4.2 mm).