The reaction of phlogopite plus quartz to enstatite, potassium feldspar, and aqueous fluid in the system KMASH-KCl was reversed at 2-12 kbar and 750-875 degrees C and at low H 2 O activities by reversal of the H 2 O content of concentrated KCl solutions equilibrated with product and reactant assemblages. Synthetic 1M phlogopite [KMg 3 AlSi 3 O 10 (OH) 2 ] and enstatite (MgSiO 3 ) maintained end-member stoichiometry throughout, and the potassium feldspar (KAlSi 3 O 8 ) was a high sanidine, based on unit-cell refinements. The broad P-T-XH 2 O and narrow reversal ranges of this investigation were possible because of the low and well-defined H 2 O activity, yet powerful fluxing action, of concentrated KCl solutions. Solubility experiments on quartz and potassium feldspar in our P-T-XH 2 O range showed that fluid-phase solution of silicate constituents was too small to have affected the H 2 O activity in the experiments. The new determinations are more definitive than previous work done at very low pressures with pure H 2 O or in CO 2 -H 2 O mixtures. They establish the standard free energy of the reaction in the experimental range with an uncertainty of about 1 kJ and indicate that the synthetic phlogopite has maximal (Al-Si) disorder under our experimental conditions. The standard enthalpy of reaction at 298 K is 106.54+ or -2.0 kJ (2sigma ) based on our reversals, a value 6 kJ less positive than that currently used by many workers in calculations of biotite stability and H 2 O activity in the petrogenesis of high-grade metamorphic rocks. The lower thermal stability that we find for phlogopite requires revision in estimates of H 2 O activity of granulite facies metamorphism: typical values for the natural assemblage orthopyroxene-biotite-garnet-potassium feldspar-plagioclase-quartz at deep-crustal metamorphic conditions (750-850 degrees C, 5-10 kbar) are alpha H2O = 0.4-0.6 compared with values of 0.15-0.30 which would have been estimated with previously available data on phlogopite stability. An important consequence of the expanded H 2 O activity range of granulites is that alkali chloride solutions of only moderate concentration [XH 2 O = 0.5-0.7], which are the values observed in actual fluid inclusions in many kinds of igneous and metamorphic rocks, are a feasible alternative to the vapor-absent conditions considered necessary by many workers based on previous low estimates of H 2 O. Participation of concentrated brines in deep-crust/upper mantle metamorphic processes enables alkali metasomatism and other kinds of chemical transport in an aqueous fluid without large-scale melting of the crust.