Numerical models of the evolution of interstellar plasmas often assume that the adiabatic parameter $\gamma$ (the ratio of the specific heats at constant pressure and volume) is constant. However, it is determined by the total internal energy of the plasma, which depends on the ionic and excitation state of the plasma. Hence, the adiabatic parameter may not be constant across the range of temperatures available in the interstellar medium.

We carry out detailed simulations of the thermal and non-thermal plasmas evolution in order to determine the temperature variability of the total internal energy and of the adiabatic parameter. The plasma, composed of H, He, C, N, O, Ne, Mg, Si, S, and Fe atoms/ions, evolves under equilibrium and non-equilibrium ionization conditions from an initial temperature of $10^9$ K. The calculations include electron impact ionization, radiative and dielectronic recombinations and line excitation. 

The total internal energy of the plasma is dominated by the ionization energy for temperatures lower than $10^5$ K with
the excitation energy having a contribution of less than one percent. In the range of temperatures from 10^3 to 10^9 K the adiabatic parameter between 1.01 and 5/3.