A hydrothermal-fluid flow, ascending from a deep-seated reservoir of primary hydrothermal fluid dominated by sodium and chloride towards the land surface, has been modeled using a Selector-C program, based upon the thermodynamic-potentials minimization, and also petrographic and geochemical data from the Pauzhetka geothermal production field (Southern Kamchatka, Russia). Speciation of volatiles has been calculated for the hydrothermal solution. The amounts of these species have been estimated under the temperature and pressure of main regions of the geothermal area. The effect of the presence of atmospheric air on the abundance of the volatiles has been studied under the conditions of the upper aquifer and the discharge of geothermal water.