Third-order nonlinear IR spectroscopy has been used to study the vibrational population relaxation and the vibrational dephasing dynamics of the CN stretching mode of Fe(CN)(6)(4-) in D2O. Transient-grating signals in the magic-angle condition show a biexponential decay upon the excitation of the T-Iu mode of the CN stretching motion. A fast-decaying component (similar to 700 fs) of the transient-grating signal is attributed to the rapid equilibration between the T-Iu mode and the Raman-active modes. The time scale of the slow-decaying component is around 23 ps, which corresponds to the vibrational population relaxation from the v = I state of the CN stretching mode. Anisotropy of the transient-grating signal decays with a time constant of 2.6 ps, which is due to the time evolution of the superposition states of the triply degenerate T-Iu modes. Three-pulse photon-echo measurements showed that the correlation function of the frequency fluctuations decays biexponentially with time constants of 80 fs and 1.5 ps. The time scales of the decay of the correlation function are similar to those for SCN- in D2O even though the coupling strength of the solute-solvent interaction depends on the systems. On the basis of these results, we discuss the vibrational dynamics of the CN stretching mode of Fe(CN)(6)(4-) in D2O.