A new differential Row mixing calorimeter for operation at high temperature and pressures is reported. The calorimeter has been used to make measurements of excess enthalpies H-m(E) for {xH(2)O + (1-x)CH3OH} over the temperature range T = 423 K to T = 523 K at pressures up to 20 MPa. The measurements are compared with those of Simonsen et al. (J. Chem. Thermodynamics 1987, 19, 479-492) and agreement to within the combined experimental error is found.