A new partition function for water dimer in the temperature range 200-500 K was developed by exploiting the equations of state for real water vapor, liquid water, and ice, and demonstrated to be significantly more accurate than any proposed so far in the literature. The new partition function allows the Active Thermochemical Tables (ATcT) approach to be applied on the available experimental and theoretical data relating to water dimer thermochemistry, leading to accurate water dimer enthalpies of formation of -499.115 +/- 0.052 kJ mol(-1) at 298.15 K and -491.075 +/- 0.080 kJ mo1(-1) at 0 K. With the current ATcT enthalpy of formation of the water monomer, -241.831 +/- 0.026 kJ mol(-1) at 298.15 K (-238.928 kJ mol(-1) at 0 K), this leads to the dimer bond dissociation enthalpy at 298.15 K of 15.454 +/- 0.074 kJ mo1(-1) and a 0 K bond dissociation energy of 13.220 +/- 0.096 kJ mol(-1) (1105 +/- 8 cm(-1)), the latter being in perfect agreement with recent experimental and theoretical determinations. The new partition function of water dimer allows the extraction and tabulation of heat capacity, entropy, enthalpy increment, reduced Gibbs energy, enthalpy of formation, and Gibbs energy of formation. Newly developed tabulations of analogous thermochemical properties for gas-phase water monomer and for water in condensed phases are also given, allowing the computations of accurate equilibria between the dimer and monomer in the 200-500 K range of temperatures.