The equilibrium constant for water dimerization (K-P) was determined as a function of temperature via rigorous calculation of the canonical (H2O)(2) partition function using the recently developed split Wigner pseudospectral method and the VRT(ASP-W) pair potential. Our K-P(T) values are significantly larger than those from previous theoretical treatments but somewhat smaller than literature experimental values, which exist, however, only over a very limited temperature range. These results indicate that water dimers can exist in sufficient concentrations (e.g., 10(16) cm(-3) at 40 degreesC and 100% relative humidity) to affect physical and chemical processes in the atmosphere.