Progressive regional metamorphism of impure carbonate rocks from south-central Maine is represented in terms of advancement of a reaction progress variable. Reaction progress serves as a quantitative monitor of (a) changes in modal mineralogy; (b) changes in rock volume; (c) changes in whole-rock chemistry; and (d) kinds and amounts of volatile species evolved by rocks during the metamorphic event. Reaction progress also monitors the rocks' interaction with external heat and fluid reservoirs and the amount of P–V work performed by the rocks on their surroundings. Because some correspondence exists between time and reaction progress, reaction progress orders various metamorphic processes in an internally-consistent chronology. As reaction progressed in the carbonate rocks: (a) muscovite, ankerite, albite, and ilmenite, initially in the rocks, were destroyed; sphene, zoisite, and diopside were created; while biotite, chlorite, quartz, calcic amphibole, anorthite, microcline, and calcite were first created and later destroyed; (b) rocks became depleted in K, Na, H, C, and O which were lost principally as the volatile species H2O, CO2, KCl, and NaCl; (c) rocks were infiltrated by up to 1·7 rock volumes H2O fluid; (d) up to 230 cal heat were added to each cm3 metamorphic rock; and (e) the rocks performed up to 12 cal/cm3 rock of P–V work on their surroundings.