Abstract   Experiments were performed in the three-phase system high-silica rhyolite melt + low-salinity aqueous vapor + hydrosaline brine, to investigate the exchange equilibria for hydrogen, potassium, and sodium in magmatic-hydrothermal systems at 800 °C and 100 MPa, and 850 °C and 50 MPa. The K ^aqm/melt _H,Na and K ^aqm/melt _H,K for hydrogen-sodium exchange between a vapor + brine mixture and a silicate melt are inversely proportional to the total chloride concentration (ΣCl) in the vapor + brine mixture indicating that HCl/NaCl and HCl/KCl are higher in the low-salinity aqueous vapor relative to high-salinity brine. The equilibrium constants for vapor/melt and brine/melt exchange were extracted from regressions of K ^{a q m / m e l t} _{H , N a} and K ^{a q m / m e l t} _{H , K} versus the proportion of aqueous vapor relative to brine in the aqueous mixture (F^aqv) at P and T, expressed as a function of ΣCl. No significant pressure effect on the empirically determined exchange constants was observed for the range of pressures investigated. Model equilibrium constants are: K ^aqv/melt _H,Na(vapor/melt)=26(±1.3) at 100 MPa (800 °C), and 19( ± 7.0) at 50 MPa (850 °C); K ^aqv/melt _H,K=14(±1.1) at 100 MPa (800 °C), and 24(±12) at 50 MPa (850 °C); K ^aqb/melt _H,b(brine/melt)= 1.6(±0.7) at 100 MPa (800 °C), and 3.9(±2.3) at 50 MPa (850 °C); and K ^aqb/melt _H,K=2.7(±1.2) at 100 MPa (800 °C) and 3.8(±2.3) at 50 MPa (850 °C). Values for K ^aqv/melt _H,K and K ^aqb/melt _H,K were used to calculate KCl/HCl in the aqueous vapor and brine as a function of melt aluminum saturation index (ASI: molar Al₂O₃/(K₂O+Na₂O+CaO) and pressure. The model log KCl/HCl values show that a change in melt ASI from peraluminous (ASI = 1.04) to moderately metaluminous (ASI = 1.01) shifts the cooling pathway (in temperature-log KCl/HCl space) of the aqueous vapor toward the andalusite+muscovite+K-feldspar reaction point.