Non-equilibrium (time-dependent) cooling rate and ionization state calculations are presented for a gas behind a shock wave with v ∼ 50–150 km s−1 (Ts∼ 0.5–6 × 105 K). Such shock waves do not lead to the radiative precursor formation; that is, the thermal evolution of a gas behind the shock wave is controlled by collisions only. We have found that the cooling rate in a gas behind a shock wave with v ∼ 50–120 km s−1 (Ts∼ 0.5–3 × 105 K) differs considerably from the cooling rate for a gas cooled from T = 108 K. It is well known that a gas cooled from T = 108 K is thermally unstable for isobaric and isochoric perturbations at K. We studied the thermal instability in a collisionally controlled gas for shock waves with v ∼ 50–150 km s−1. We found that the temperature range within which the post-shock gas is thermally unstable is significantly modified and depends on both gas metallicity and the ionic composition of the gas before the shock wave. For , the temperature range for which the thermal instability criterion for isochoric perturbations is not fulfilled widens in comparison with that for a gas cooled from T = 108 K, while that for isobaric perturbations remains almost unchanged. For Z ∼ Z⊙, the gas behind a shock wave with km s−1 ( K) is thermally stable to isochoric perturbations during all of its evolution. We have shown that the transition from isobaric to isochoric cooling for a gas with behind a shock wave with Ts= 0.5–3 × 105 K occurs in a gas layer column density layer behind a shock wave than that for a gas cooled from T = 108 K. The ionic states in a gas with Z ∼ 10−3–1 Z⊙ behind shock waves with K demonstrate a significant difference from these in a gas cooled from T = 108 K. Such a difference is thought to be important for the correct interpretation of observational data, but is not very helpful for discriminating thermally stable gas.