Flow calorimetric measurements of the excess molar enthalpy H-m(E) of {xH(2)O+(1-x)C2H5OH} at the pressure 15 MPa and over the temperature range 398 K to 548 K are reported. The accuracy of the measurements is 2 per cent at the lower end of the temperature range and 3 per cent at the higher end. The measurements are in good agreements with those of Ott et al. (J. Chem. Thermodynamics 1987, 19, 337-348) who recommended that this mixture should be adopted as a reference system for the testing of how mixing calorimeters. A simple three parameter equation which gives the correct shapes of the H-m(E)(x) curves at all temperatures is suggested. The equation fits the measurements at high temperatures to within the accuracy of the experiments; in the region x > 0.5, where H-m(E) changes sign and the curve becomes S shaped the fit is less good, but at lower temperatures the fit is again good. The three parameters have been fitted to polynomials in powers of the temperature so that the shape of the H-m(E) (x,T) surface at p = 15 MPa can be calculated over the full temperature range from T = 298 K to T = 548 K.