The temperature dependence of the heat capacity of deuterium oxide (99.9 at% of deuterium) in both crystalline and liquid state was determined from T = (6 to 350) K by adiabatic vacuum calorimetry, mainly with an uncertainty of +/-0.2%. The melting temperature and the enthalpy of fusion were measured, while the entropy of fusion was estimated. The experimental data were used to calculate following standard thermodynamic functions of deuterium oxide in the same temperature range: C-p,m(o)/R,Delta H-T(o)m(o)/R,Delta S-T(o)m(o)/R and Phi(o)(m) = Delta S-T(o)m(o) -Delta H-T(o)m(o)/T (where R is the universal gas constant). These results were compared with previously published literature data. (C) 2005 Elsevier Ltd. All rights reserved.