A fundamental equation of state has been formulated for heavy water in the form &PSgr; = &PSgr;(p,T) in which &PSgr; = Helmholtz free energyp= densityT= thermodynamic temperature. The complete range of single phase states in the range up to 100 MPa and 600 °C is covered by a single equation which is fitted both toPvTvalues, for saturated and unsaturated states, and to enthalpy values for saturation states only. The equation is constrained to fit the critical point conditions determined by Blank. It represents all thermodynamic properties of D2O, in the above range of states, within what is believed to be the accuracy of the experimental data.