An accurate equation of state is presented for the thermodynamic properties of mixtures of H2O and D2O in the critical region. The equation incorporates a crossover from fluctuation-induced singular behavior near the locus of vapor-liquid critical points to regular behavior outside the critical region. It is a generalization of a crossover equation-of-state previously developed for pure H2O and pure D2O extended to the mixtures on the basis of the principle of isomorphism of critical behavior. The equation represents the thermodynamic properties of mixtures of H2O and D2O in the same range of temperatures and densities as for the pure components. A comparison is made with available experimental data for the pressure, for the isochoric heat capacity, for the enthalpy of mixing, and for the isotope fraction ratio of mixtures of H2O and D2O. An analysis is presented of the near-critical behavior of various thermodynamic properties, such as the isobaric heat capacity, the partial molar volumes, Henry's constant, and the vapor-liquid distribution ratio (K factor).