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.