The effect of melt/rock reaction on the mechanisms and kinetics of melt migration is investigated through a series of melt migration experiments in which melt saturated only in olivine infiltrates synthetic peridotites containing different proportions of olivine and Ca-poor pyroxene. The microstructures that develop are significantly different from those formed during melt migration in a chemically equilibrated system. A reaction zone develops in which all pyroxene is dissolved, the melt fraction is increased, and olivine grain size is increased. The reaction front is unstable so that finger-like structures or channels with higher melt content develop. The morphology of the instability depends on the initial pyroxene content of the infiltrated rock.This reactive-infiltration instability with the resulting formation of melt fingers could facilitate the transition from porous to channelized flow in the mantle. ¿ American Geophysical Union 1994