Permeability was measured during decarbonation reaction of calcite + quartz → wollastonite + CO2 in the presence of water. The aggregate used for measurement was initially composed of 90 wt% calcite and 10 wt% quartz. At an effective pressure (confining pressure minus pore fluid pressure) of 25 MPa, the reaction proceeds to completion in <40 h at temperatures of 600–700 °C. Permeability remains nearly constant at about 10−16.5 m2 at 600 °C, but it evolves from 10−16.5 to 10−19 m2 at 700 °C as quartz and calcite react to form porous wollastonite aggregates. Over the same temperature range, but at an effective pressure of 100 MPa, the permeability reduces rapidly below 10−19 m2, and only limited volumes of dense wollastonite aggregates were produced on calcite grain surfaces. Our experiments demonstrate the importance of effective pressure in influencing reaction progress and permeability evolution during metamorphic reactions. The experiments clearly suggest that where water-infiltration–driven decarbonation in siliceous carbonates occurs under near-lithostatic fluid pressure conditions, reaction kinetics can be very fast and the release of carbon dioxide is continuous.