Average D¿ values for NaSi-CaAl interdiffusion in the compositional interval from An0 to An26 have been determined at 1000¿ C by the method of lamellar homogenization. At 1500 MPa confining pressure (P), D¿ increases 1 order of magnitude (5.0¿10-21 to 4.0¿10-20 m2/s) for 4 orders of magnitude increase in hydrogen fugacity (0.029 Mn3O4-Mn2O3 buffer, to 197 MPa FeO-Fe3O4 buffer). At constant hydrogen fugacity (fH2), D¿ increases rapidly at low pressure and becomes nearly independent of P above 1000 MPa. (For fh2=0.1 MPa, D¿=2.8¿10-22 m2/s at P=0.1 MPa, 5.0¿10-21 at P=500, 1.3¿10-20 at P=1000, and 1.4¿10-20 at P=1500). The dependence of D¿ on increasing pressure, when a hydrogen-related species is present, is believed to be due to an increase in the concentration of the structural defect associated with increase in the hydrogen impurity. In most crustal igneous rocks, which are internally buffered near quartz-fayalite-magnetite, the dependence of D¿ on fH2 is relatively minor compared to the effect of confining pressure. ¿ American Geophysical Union 1989