Abstract A method is presented for calibrating flow heat-capacity calorimeters that is a factor of 10 more precise than that previously used. Comparison of the two methods shows that they give the same results if corrections are made for differences in experimental conditions. Calibration corrections for the calorimeter were found to depend strongly on the heat capacity of the solution being measured. Measurements of the heat capacity of NaCl(aq), from 1 to 6 mol · kg −1, are reported to 598 K at 20 MPa and at 523 K to 36 MPa. Comparison with literature values shows excellent agreement between 453 and 573 K, prompting recommendation of reference values for the heat capacity of concentrated NaCl(aq). Literature predictions of the pressure dependence of the apparent molar heat capacity are generally in poor agreement with those measured at 36 MPa.