To investigate channelization of a reactive melt in mantle rocks, we imposed a gradient in fluid pressure across a partially molten rock composed of olivine and clinopyroxene, sandwiched between a source of alkali basalt melt and a sink of porous alumina. We performed experiments at a confining pressure of 300 MPa and pore pressures of 0.1–300 MPa, resulting in fluid pressure gradients of 0–88 MPa/mm at temperatures of 1200–1250 °C. When the gradient in fluid pressure is zero, only a planar reaction layer composed of olivine + melt develops, in agreement with previous experiments. However, if the gradient in fluid pressure is greater than zero, in addition to the planar reaction layer, finger-like melt-rich channels that contain olivine + melt develop and propagate into the rock, significantly past the interface between the melt reservoir and the partially molten rock. Channelization of the melt results in a significant increase in permeability and hence in the flux of melt through the partially molten rock.