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The heat capacity of heavy water was measured in the temperature range from 294 to 746 K and at densities between 52 and 1,105 kg{center_dot}m{sup {minus}3} using a high-temperature, high-pressure adiabatic calorimeter. The measurements were performed at 14 liquid and 9 vapor densities between 52 and 1,105 kg{center_dot}m{sup {minus}3}. Uncertainties of the measurements are estimated to be within 3% for vapor isochores and 1.5% for the liquid isochores. In the region of the immediate vicinity of the critical point (0.97 {le} T/T{sub c} {le}1.03 and 0.75 {le}p/p{sub c} {le}1.25), the uncertainty is 4.5%. The original C{sub v} data were corrected and converted to the new ITS-90 temperature scale. A parametric crossover equation of state was used to represent the isochoric heat capacity measurements of heavy water in the extended critical region, 0.8 {le} T/T{sub c} {le}1.5 and 0.35 {le}p/p{sub c} {le}1.65. The liquid and vapor one- and two-phase isochoric heat capacities, temperatures, and saturation densities were extracted from experimental data for each measured isochore. Most of the experimental data are compared with the Hill equation of state, and the overall statistics of deviations between experimental data and the equation of state are given.