Excess molar enthalpies of aqueous solutions of diethanolamine, methyldiethanolamine, ethyldiethanolamine, and n-butyldiethanolamine have been measured at temperatures from (298.15 to 338.18) K. Excess molar enthalpies of these mixtures depend on the size of the alkyl groups attached to the nitrogen atom of the alkyldiethanolamine. A similar effect has been found for the excess molar volumes of these aqueous mixtures.