The heat of micellization DeltaH(m), for five nonionic surfactants (C10E5, C10E6, C10E7, C10E8, C8E5) was measured as a function of temperature by using an isothermal titration calorimeter. The abbreviation CiEj stands for the nonionic surfactant CH3(CH2)(i-l)(OCH2CH2)(j)OH. The heat capacity change upon micellization DeltaC(p) was determined from DeltaH(m)'s. It is found that DeltaC(p) is constant for four C10Ej's. That is DeltaC(p) is independent of the chain length of hydrophilic headgroups. Furthermore, DeltaC(p) is directly proportional to the removal of water accessible nonpolar surface area DeltaA(np). The ratio DeltaC(p)/DeltaA(np) is equal to -1.28 +/- 0.06 J mol(-1) K-1 Angstrom(-2) for C10Ej's and -1.34 +/- 0.06 J mol(-1) K-1 Angstrom(-2) for C8E5, consistent with that observed upon protein folding (-1.34 +/- 0.33 J mol(-1) K-1 Angstrom(-2)) of Spolar et al. (Biochemistry 1992, 31, 3947.).