Veins in metamorphic rocks result from the interplay of two processes: deformation and mineral reactions. Two kinds of veins have been recognized: segregation and hydrofracturing. This paper examines quantitatively the conditions for hydrofracturing and associated vein formation due to devolatilization reactions. The inflection point of the reaction curve defines a boundary between the reaction-enhanced ductility regime (high-pressure side) and the hydrofracturing regime in a pressure-temperature diagram. Low strain rates and low permeability of rocks, together with the irreversible reaction, are important factors for hydrofracturing. Combination of reaction kinetics and percolation theory enables us to estimate the time required for macroscopic hydrofracturing. Model calculations are carried out at 1 kbar for the reaction serpentine + brucite = forsterite + water. It takes several tens of years for macroscopic hydrofracturing to develop. The result depends on rock texture (the lattice type in percolation theory) and superheating degrees; however, hydrofracturing will generally not exceed 200 yr.