Extraction of melt is influenced by the rheology of the two phase medium in which it is generated. The viscosity of mantle containing a small melt fraction is significantly increased as the concentration of water dissolved in olivine decreases. Upon melting, water fractionates into melt, where it is transported by both advection and diffusion. Advection transports water into melt rich regions, resulting in higher olivine water contents, lower solid viscosity, and lower pressure locally. Melt flows into these low pressure regions, resulting in self-induced localization of melt. Diffusive transport of water opposes advection, creating a preferred wavelength of melt localization. An estimated wavelength of melt localization in the range of 100's of meters to a few kilometers may lead to the formation of high melt fraction channels which preserve chemical disequilibrium between melt and residual mantle. ¿ 2000 American Geophysical Union