We determined the kinetics of Fe-Mg interdiffusion in ringwoodite aggregates as a function of water content (up to 6,000wt.ppm H2O) at 20GPa and 1,373-1,673K by the diffusion couple method. The dependence of Fe-Mg interdiffusivity (DFe-Mg) on Fe concentration was determined using the Boltzmann-Matano method. The experimentally reported DFe-Mg in ringwoodite within 0X(Fe)0.1 could be fitted by the relation where E*=(1-X-Fe)E-Mg+XFeEFe(E-Mg=1405kJ/mol, E-Fe=42kJ/mol), D-0 = 5.59-1.91+2.90x10(-10)m(2)/s, n=-0.210.10, r=0.250.03, and =-24 +/- 4. The water content exponent r of 0.25 suggests a nonnegligible role of water in enhancing Fe-Mg interdiffusion in ringwoodite. The length scale over which the chemical heterogeneities are homogenized by Fe-Mg interdiffusion in the mantle transition zone is estimated to be only a few hundred meters even assuming the whole Earth age. Comparison between the conductivities predicted from Fe-Mg interdiffusion and those obtained from magnetotelluric surveys suggests that around 0.1wt.% water can account for the high conductivity anomalies (10(-0.6)-10(-1)S/m) observed in the lower part of the mantle transition zone.