Methods previously used to study hydrophobic interaction in ordinary water have been used to measure the free energy of hydrophobic interaction between hydrocarbon chains in deuterium oxide at 25�: ��� (1) Conductance measurements on micellar solutions of a series of hexadecyltrimethylammonium carboxylates give the effect of hydrocarbon chain length on the free energy of binding a counter-ion (carboxylate ion) to the micelle, and hence the free energy of hydrophobic interaction. ��� (2) Comparison of the kinetics of the reaction between decylamine and 4-nitrophenyl decanoate with the kinetics of the corresponding reaction without hydrophobic side chains (the reaction between ethylamine and 4-nitrophenyl acetate) gives the extent to which hydrophobic interaction stabilizes the transition state. This then provides another estimate of the free energy of hydrophobic interaction. ��� The free energy of hydrophobic interaction in D2O was found to be - 1.76 kJ mol-1, for each contact between two methylene groups, compared with -1.40 kJ mol-1 in H2O. Hydrophobic interaction is therefore stronger in D2O than in H2O.