The heat capacity of heavy water was measured in the temperature range from
294 to 746 K and at densities between 52 and 1105 kg.m(-3) using a high-te
mperature, high-pressure adiabatic calorimeter. The measurements were perfo
rmed at 14 liquid and 9 vapor densities between 52 and 1105 kg.m(-3). Uncer
tainties of the measurements are estimated to be within 3% for vapor isocho
res and 1.5% for the liquid isochores. In the region of the immediate vicin
ity of the critical point (0.97 less than or equal to T/T-c less than or eq
ual to 1.03 and 0.75 less than or equal to rho/rho(c) less than or equal to
1.25), the uncertainty is 4.5%. The original C-V data were corrected and c
onverted to the new ITS-90 temperature scale. A parametric crossover equati
on of state was used to represent the isochoric heat capacity measurements
of heavy water in the extended critical region, 0.8 less than or equal to T
/T-c less than or equal to 1.5 and 0.35 less than or equal to rho/rho(c) le
ss than or equal to 1.65. The liquid and vapor one- and two-phase isochoric
heat capacities, temperatures, and saturation densities were extracted fro
m experimental data for each measured isochore. Most of the experimental da
ta are compared with the Hill equation of state, and the overall statistics
of deviations between experimental data and the equation of state are give
n.