The PVTx relationships of H2O+D2O mixtures were measured in the near-critical and supercritical regions. Measurements were made using a constant-volume piezometer immersed in a precision thermostat. The volume of the piezometer V-PT was corrected for both temperature and pressure expansions. The uncertainty of the density measurements was estimated to be 0.05-0.10%, depending on the experimental pressure and temperature. The uncertainty of the temperature, pressure, and composition measurements were respectively 10 mK, 0.05%, and 0.001 mol fraction. Measurements were made along various near-critical and supercritical isotherms between 517 and 680 K at pressures from 3 to 38 MPa and densities from 97 to 466 kg m(-3) for two compositions, namely 0.5 and 0.6 mol fraction of H2O. The measured PVTx data for H2O+D2O mixtures were compared with values calculated from a six-term Landau expansion and from parametric crossover equations of state. The accuracy of the method was confirmed by PVT measurements for pure water in the critical and supercritical regions. The values of partial molar volumes for dilute H2O+D2O mixtures near the critical point of pure H2O and the Krichevskii parameter were calculated using a parametric crossover equation of state and the critical locus data. Using measured values of molar volumes and corresponding values for pure components, we derived the values of excess molar volumes as a function of pressure for the supercritical isotherms. (C) 2002 Elsevier Science B.V. All rights reserved.