Heat capacities of aqueous mixtures of monoethanolamine with N-methyldiethanolamine were measured from 30 to 80 degreesC with a differential scanning calorimeter (DSC). The binary systems studied were N-methyldiethanolamine + water and monoethanolamine + N-methyldiethanolamine. For mole fractions of water ranging from 0.2 to 0.8, 16 concentrations of the mono ethanolamine + N-methyldiethanolamine + water systems were studied. The estimated uncertainty of the measured heat capacities is +/-2% including the effect of up to 1% impurities in a substance. An excess molar heat capacity expression using the Redlich-Kister equation for the composition dependence is used to represent the measured C-p of aqueous alkanolamine solutions. The overall average absolute percentage deviation of the calculation of the molar heat capacity for a total of 176 data points for the monoethanolamine + N-methyldiethanolamine + water system is 0.5%. The heat capacities of aqueous mixtures of monoethanolamine with N-methyldiethanolamine presented in this study are, in general, of sufficient accuracy for most engineering-design calculations.