Enthalpies of mixing of isomolal Na2CO3 and NaHCO3 aqueous solutions were measured in the 0.1-1.0 molality and 293-328 K temperature range using a flow-mix microcalorimetric cell. Heat capacities of the mixture were calculated from a slope of the dependence of the enthalpy of mixing on temperature, and the values obtained varied in the range from -0.019 to -0.105 Jkg(-1)K(-1). The heat capacity versus molality curve shows a minimum corresponding to the electrolyte molality for which complete ion hydration is attained.