Heats of mixing of Na2CO3 and NaHCO3 aqueous solutions, of the same molal ionic strength, were measured in the 0.02-1.2 m (molality) range and at 298 K using a flow-mix microcalorimetric cell. The most concentrated solutions, 0.4 m Na2CO3 and 1.2 m NaHCO3, were mixed at 298, 308, 318, and 328 K; the maximum heat of mixing value was obtained at 308 K. The results are interpreted in terms of the solute-water structural properties.