Values for the heat capacities of solid deuterium oxide and the resulting liquid, from initial temperatures between 4 °C. and − 78.5 °C. to a final temperature of 25.0 °C, have been determined. The specific heats of the solid over the temperature range 0 to − 70 °C. have been measured. The latent heat of fusion of deuterium oxide has been determined. The specific heat of liquid deuterium oxide is shown to be greater than that of water in the temperature region in which measurements were made. A comparison of the thermal properties of deuterium oxide with those of hydrogen oxide has been made, and certain points of interest are indicated.