The heat capacities (C(p2)degrees,) of pure liquid nonionic surfactants Triton X-100 and n-decyl and n-dodecyl poly(oxyethylene glycol) monoethers C(10)E(5), C(10)E(6), C(10)E(8), C(12)E(5), C(12)E(6), and C(12)E(8) have been measured using a differential scanning calorimeter. The partial molar heat capacities of these surfactants in water in the post micellar state ((C) over bar(p2,mic)) have been evaluated from apparent molar heat capacities (C-p phi) which are measured using a Picker Microflow calorimeter. The standard partial molar heat capacities ((C) over bar(p2)degrees) have been obtained (i) by p combining the heat capacities of the pure surfactants (C(p2)degrees) with the heat capacities of dissolution in the premicellar (Delta C(p,mono)degrees) region and (ii) by combining the partial molar heat capacities in the post micellar state ((C) over bar(p2,mic)) with the heat capacities of micellization (Delta C-p,C-m). The comparison of the (C) over bar(p2)degrees estimated by using a group additivity rule with the experimental values indicates that the additivity rule for the estimation of the standard partial molar heat capacities in water holds reasonably well for nonionic surfactants for which it is difficult to obtain accurate (C) over bar(p2)degrees experimentally due to their low critical micelle concentration (cmc).