The heat capacity of Cd(H2O)(2)Ni(CN)(4) . 4H(2)O crystal in which the water molecules form a two-dimensional quasi-hexagonal hydrogen-bond network was measured with an adiabatic calorimeter in the temperature range 5-300 K, A glass transition due to the freezing of the reorientational motion of the water molecules was found around 120 K. The reorientational relaxation times were obtained from the enthalpy relaxation data around the glass transition. The complex dielectric permittivity was also measured in the temperature range 150-285 K and in the frequency range 20 Hz-1 MHz. A dielectric dispersion associated with the reorientational motion of the water molecules was observed in the temperature range 160-278 K. The dielectric relaxation rimes were calculated from the temperature and frequency dependence of the epsilon " data, Calorimetric and dielectric relaxation times lie on separate smooth lines in the wide time scale over 10(-6)-10(6) s in the Arrhenius plot. The mechanism of the water reorientational motion was discussed on the basis of the model considering the formation and transfer of the Bjerrum defect as in the case of the three-dimensional ice polymorphs.