We review petrologic observations of reaction textures from high-grade rocks that suggest the passage of fluids with variable alkali activities.Development of these reaction textures is accompanied by regular compositional variations in plagioclase,pyroxenes,biotite,amphibole and garnet.The textures are interpreted in terms of exchange and net-transfer reactions controlled by the K and Na activities in the fluids.On the regional scale,these reactions operate in granitized,charnockitized.syenitized etc.shear zones within high-grade complexes.Thermodynamic calculations in simple chemical systems show that changes in mineral assemblages,including the transition from the hydrous to the anhydrous ones.may occur at constant pressure and temperature due only to variations in the H_2O and the alkali activities.A simple procedure for estimating the activity of the two major alkali oxides,K_2O and Na_2O,is implemented in the TWQ software Examples of calculations are presented for well-documented dehydration zones from South Africa.southern India.and Sri Lanka.The calculations have revealed two end-member regimes of alkalis during specific metamorphic processes:rock buffered,which is characteristic for the precursor rocks containing two feldspars,and fluid-buffered for the precursor rocks without K-feldspar.The observed reaction textures and the results of thermodynamic modeling are compared with the results of available experimental studies on the interaction of the alkali chloride and carbonate-bearing fluids with metamorphic rocks at mid-crustal conditions.The experiments show the complex effect of alkali activities in the fluid phase on the mineral assemblages.Both thermodynamic calculations and experiments closely reproduce paragenetic relations theoretically predicted by D.S.Korzhinskii in the1940 s. We review petrologic observations of reaction textures from high-grade rocks that suggest the passage of fluids with variable alkali activities. Development of these reaction textures is accompanied by regular compositional variations in plagioclase, pyroxenes, biotite, amphibole and garnet.The textures are interpreted in terms of exchange and net-transfer reactions controlled by the K and Na activities in the fluids.On the regional scale, these reactions operate in granitized, charnockitized.syenitized etc.shear zones within high-grade complexes. Hermodynamic calculations in simple chemical systems show that changes in mineral assemblages, including the transition from the hydrous to the anhydrous ones. occur occur at constant pressure and temperature due only to variations in the H_2O and the alkali activities. A simple procedure for estimating the activity of the two major alkali oxides, K_2O and Na_2O, is implemented in the TWQ software Examples of calculations are presented for well-documented dehydration zones from South Africa. southern India. and Sri Lanka. These calculations have revealed two end-member regimes of alkalis during specific metamorphic processes: rock buffered, which is characteristic for the precursor rocks containing two feldspars, and fluid-buffered for the precursor rocks without K-feldspar. The observed reaction textures and the results of thermodynamic modeling are compared with the results of available experimental studies on the interaction of the alkali chloride and carbonate-bearing fluids with metamorphic rocks at mid-crustal conditions. experiments demonstrated the complex effect of alkali activities in the fluid phase on the mineral assemblages. But thermodynamic calculations and experiments closely reproduce paragenetic relations theoretically predicted by DS Korezhinskii in the1940 s.