In the present work, the linear hydrodynamic stability theory of infinitely compressible mixing layers at high temperatures, where the gas behaves as thermally ideal, has been treated. The basic flow is not homentropic, so that the velocity and temperature profiles are independent of each other, with the influence of the symmetry or asymmetry of these profiles on the stability properties being discussed in more detail. The general stability criteria were derived and some problems were solved numerically. The influence of basic parameters, such as Mach number, mixing layer thickness, temperature excess or deficit in the critical layer and others, on the position of the neutral boundary and on the growth factor of unstable modes was studied.