Vapor pressure data of geologically and industrially important aqueous solutions containing two or more electrolytes are lacking, especially at high temperatures (>373 K). In this study, a method was developed to measure the vapor pressures of aqueous solutions containing both NaC1 and KC1, with molalities of (1) 0.5 mol/kg NaC1 and 0.5 mol/kg KCl, (2) 1 mol/kg NaCl and 1 mol/kg KCl, and (3) 2 mol/kg NaCl and 2 mol/kg KCl, at temperatures from 493.15 to 693.25 K. This method was verified by comparing our measured vapor pressures of pure H2O with the previously well -established data and was further tested by comparing our measured vapor pressures of aqueous solutions containing either NaCl or KCl at 1 and 3 mol/kg concentrations with previous data and models. This study not only provided the vapor pressure data for the ternary NaCl-KCl-H2O system at elevated temperatures for the first time but also established a reliable experimental method for vapor pressure measurements of multicomponent aqueous solutions commonly involved in various geological and industrial processes at elevated temperatures.