Abstract—Water injection into a high-temperature geothermal reservoir saturated with superheated vapor is investigated. The stability of the phase transition front moving at constant speed is investigated by the normal modes method. Numerical study of the obtained dispersion equation is carried out. At different values of the main parameters, the dispersion curves, characterizing the transition to instability are presented. It is shown, that transition from the stable front regime to the unstable one occurs with the increase in the permeability and initial temperature, and also with the decrease in the initial pressure.