Fluid immiscibility between a CO2-rich fluid and brine has been described from several medium- to high-grade marbles and calcsilicates. Here we also report evidence for CH4-H2O immiscibility in low-grade metamorphic rocks from Wales and suggest that the flow of such immiscible fluids may be comparable to that of water-oil or water-gas mixtures in petroleum reservoirs. This analogy implies that where one fluid is being produced by reaction it will flow out of the rock while the other remains immobile, except insofar as it is soluble in the more abundant fluid phase. Hence, extensive reaction may take place in the presence of coexisting immiscible fluids, each having high activities. It is proposed that this type of flow behavior not only accounts for the development of salt-saturated fluids in marbles but also may be responsible for the elimination of carbonate and extensive loss of graphite from pelites in the lower part of the greenschist facies.