Mineral reactions at the biotite isograd were investigated in pelitic schists, micaceous sandstones, micaceous limestones, and metaigneous rocks from three stratigraphic units over an area of ∼10000 km2 in north-central New England. The biotite isograd in north-central New England represents a metamorphic decarbonation front that affected all major rock types in each stratigraphic unit. Pressure at the isograd was near 3500 b at the northern end of the study area and near 5500 b in the south. Temperature was in the range 400–450°C. Equilibrium metamorphic fluids were approximately CO2-H2O mixtures with XCO2=0·04–0·07. Volumetric fluid]-rock ratios were calculated for more than 70 samples of all major rock types from each formation using measured progress of the prograde reactions and the estimated P-T-XCO2, conditions of metamorphism. Regardless of stratigraphic unit, limestones record low values of 0–0·2, pelites and metaigneous rocks generally record high values of ∼1–3, and standstones record intermediate values of ∼0·2–1. With exception of the limestones, all samples from the biotite zone record fluid-rock ratios significantly greater than likely rock porosity during metamorphism. The prograde decarbonation reactions therefore were driven by infiltration of rock by reactive aqueous fluids. The observed correlations between fluid-rock ratio and rock type indicate that significant permeability contrasts occurred during low-grade metamorphsim with permeability increasing in the order: limestonesa, 1988) demonstrate that low-grade metamorphism over much of the northern Appalachian orogen was infiltration-driven.