This study is aimed at determining the diffusion coeffcient of net-work modifiers(mainly Na, K, and Ca) in a two-phase melt-NaCl system, in which the melts are granitic andthe system is NaCl-rich in composition. The diffusion coefficients of Na, K, and Ca were measured at the temperatures of 750 -1400℃, pressures of 0. 001×10~8 - 2×10~8 Pa, and initialH_2O contents of O wt% - 6. 9 wt% in the granitic melts. The diffusion coefficients of Fe andMg were difficuIt to resolve. In all experiments a NaCl melt was present as well. In the absence of H_2O, the diffusion of net-work modifiers folows an Arrhanious equation at 1 ×10~5 Pa:lgD_Ca= - 3. 88 - 5140/ T, lgD_K = - 3. 79 - 4040/ T, and lgD_Na=- 4.99 - 3350/ T,where D is in cm~2/s and T is in K. The diffusion coefficients of Ca, Na, K, and Fe increasenon-linearly with increasing H_2O cOntent in the melt. The presence of about 2 wt% H_2O inthe melt will lead to a dramatical increase in diffusivity, but higher H_2O content has only a minor effect. This change is probably the result of a change in the melt structure when H_2O ispresent. The diffusion coefficients measured in this study are significantly different from thosein previous woks. This may be understood in terms of the “transient two-liquid equilibrium”theory. Element interdiffusion dapends not only on its concentration, but also on its activity coefficient gradient, which is reflected by the distribution coefficient, of the two contactingmelts. The study is aimed at determining the diffusion coeffcient of net-work modifiers (mainly Na, K, and Ca) in a two-phase melt-NaCl system, where the melts are granitic and the system is NaCl-rich in composition. The diffusion coefficients of Na, K, and Ca were measured at the temperatures of 750 -1400 ° C, pressures of 0.001 × 10 -8 to 2 × 10 -8 Pa, and initialH 2 O contents of 0 wt% -6.9 wt% in The diffusion coefficients of Fe and Mg were difficuIt to resolve. In the absence of H 2 O, the diffusion of net-work modifiers folows an Arrheni equation at 1 × 10 ~ 5 Pa : lgD_Ca = -3.88-5140 / T, lgD_K = -3.79-4040 / T, and lgD_Na = -4.993-3350 / T, where D is in cm ~ 2 / s and T is in K. The diffusion coefficients of Ca, Na, K, and Fe increasenon-linearly with increasing H_2O content in the melt. The presence of about 2 wt% H_2O inthe melt will lead to a dramatical increase in diffusivity, but higher H_2O content has only a min This change is probably the result of a change in the melt structure when H_2O is present. The diffusion coefficients measured in this study are significantly different from those previous previous woks. This may be understood in terms of the “transient two-liquid equilibrium ”theory. Element interdiffusion dapends not only on its concentration, but also on its activity coefficient gradient, which is reflected by the distribution coefficient, of the two contactingmelts.