The deuterium spin-lattice relaxation time, T 1 , of H 2 O/D 2 O mixtures is measured at 298.2 K. The relaxation rate, T 1 -1 , is found to increase with increasing deuterium atom fraction, n, the plot of T 1 -1 vs. n exhibiting a small depature from linearity. A general equation T 1 -1 (n) for the H 2 O/D 2 O system is formulated. The temperature dependence of T 1 is investigated in the temperature range 278.2 K to 298.2 K for n = 6.8 x 10 -3 , 6.8 x 10 -2 , 0.244, 0.500, and 0.997. On the assumption that the electric field gradient parameters (e 2 q Q/h and δ) are independent of n and temperature, an effective correlation time, τ c,eff , is derived from the T 1 data. Relatively large isotope effects on τ c,eff are found; possible reasons for the existence of such isotope effects are discussed in terms of a simple Debye model. The mean activation enthalpy (Δ ≠ H) and entropy (Δ ≠ S) for the relaxation process within the temperature range studied are derived on the basis of Eyring's absolute rate theory and the temperature dependence of τ c,eff . Both activation parameters are found to increase linearly with n: Δ ≠ H/kJ mol -1 = 18.2 + 2.46 n, Δ ≠ S/J K -1 mol -1 = 37.5 + 6.77 n.