A model is developed that is capable of describing the evolution of gain resulting from both rapid radiative and expansion cooling of a recombining, freely expanding plasma. It is demonstrated for the particular case of a carbon–selenium plasma that the cooling rate that leads to optimal gain can be achieved by adjusting the admixture of an efficiently radiating material (selenium) in the gain medium (carbon). Comparison is made with a recent observation of gain in an experiment done by a group from the Naval Research Laboratory and the University of Rochester, with carbon–selenium plasma for the n = 3 → 2 transition in C vi occurring at 182 A. The predicted maximum gain is ~10 cm−1 as compared with the observation of 2–3 cm−1.