The presence of hydrogen dissolved within iron- magnesium oxides and silicates results in an increase in the rate of Fe - Mg interdiffusion. Experimental data and point defect models suggest that the increased interdiffusivity is due to an increase in the total metal- vacancy concentration through stabilization of proton- vacancy defect associates in a hydrous environment. In the case of (Mg1-xFex)O, interdiffusion experiments under hydrothermal conditions at a fluid pressure of similar to 0.3 GPa yield similar dependencies of interdiffusivity on Fe- content, oxygen fugacity, and temperature as under dry conditions, but interdiffusion coefficients are a factor of similar to 3 larger. These data suggest that the increased interdiffusivities in (Mg1-xFex) O result from incorporation of defect associates formed between a metal vacancy and a single proton, P'(Me) equivalent to {p(center dot) -V ''(Me)}'. For (Mg1-xFex)(2)SiO4, interdiffusion under hydrothermal conditions over a range of fluid pressures reveals a significant difference in the dependence of interdiffusivity on Fe content than obtained under dry conditions, combined with a strong dependence on water fugacity. These data indicate that the increased diffusivities in (Mg1-xFex)(2)SiO4 result from incorporation of defect associates involving a metal vacancy and 2 protons, (2p)(Me)(x) equivalent to {2p(center dot) - V ''(Me)}. It is anticipated that, at higher water fugacities, Fe - Mg interdiffusion in both materials will become dominated by these latter defects and that the interdiffusivity will increase linearly with water fugacity but will be independent of oxygen fugacity and iron concentration.