Abstract The apparent molar heat capacities and volumes of surfactants having the general formula R(CH 3) 2PO, where R equals C 4, C 6, C 8, C 10, and C 12, tributylphosphine oxide, and octyl (CH 3) 2AsO were measured in water at 25°C. The enthalpies of dilution were also determined for hexyldimethylphosphine oxide. These data are analyzed with a mass-action model and compared with previously published data on corresponding amine oxides. For the three families of surfactants having the same chain lengths, the changes in volume and heat capacity during micellization are of the same magnitude, supporting the idea that these changes primarily reflect the loss of hydrophobic hydration of the alkyl chains upon micellization. On the other hand, the values of V 2 0 and C P,2 0, at infinite dilution, decrease, and the CMC increases, in the order PO, AsO, NO, in agreement with the increase in polarity of the head groups. The aggregation numbers of the phosphine and arsine oxides are about equal to and larger than those of the corresponding amine oxide. The values of V 2 0 of symmetrical surfactants (e.g., Bu 3PO) are smaller by about 4 cm 3 mole −1 and those of C P,2 0 are larger by about 40 J K −1 mole −1 than the values of the corresponding linear isomers (e.g., C 10PO). Tentative explanations for these trends are presented.