The effect of water on the kinetics of Fe-Mg interdiffusion in garnet was investigated at 3 GPa and 1373-1673 K using diffusion couples of pyrope and almandine aggregates, over a wide range of water content (C-H2O) from < 7 up to similar to 1260 wt. ppm. Diffusion profiles were measured by electron microprobe, and the obtained data were fitted by the Boltzmann-Matano equation. Our results show that Fe-Mg interdiffusion coefficient (DFe-Mg) is nearly independent of the composition of garnet over a wide range of Fe/Mg ratios (with X-Fe ranging from 0 to 1). The determined DFe-Mg can be described by the following Arrhenius relation: DFe-Mg (m(2)/s) = D0CH2Or exp(- Delta H/RT), where log(D-0) = -1.70 +/- 0.50, r = 1.38 +/- 0.06, and H = 310 +/- 33 kJ/mol. The exponent r of 1.38 implies the incorporation mechanism of water is hydrogen associated with metal and partly silicon vacancies to form neutral point defect complexes. While at the same time, it also suggests a profound role of water in enhancing the Fe Mg interdiffusion in garnet. With increasing water content from 100 to 1260 wt. ppm H2O, the Fe-Mg inter-diffusivity is enhanced by about two orders of magnitude, comparable to the effect of temperature increased by about 300 degrees C under dry condition. Such large effect of water on the kinetics of garnet is expected to have important implications for evaluating the closure temperature of available geospeedometry and geothermometry.