Abstract The mixed alkali iodide K 0.8Rb 0.2I has been bombarded with 13.6 MeV/A argon ions over a range of temperatures. Unlike KI and RbI that show class-I behaviour at low temperature under energetic heavy ion bombardment, K 0.8Rb 0.2I displays distinctly different behaviour. At 20 K, the F centre production is totally suppressed and is negligible at 100 K; the formation of F +–I centre pairs would only in part account for this effect. The overall production rate of defects at 20 K is larger than that at 100 K. During the lowest temperature bombardment at 20 K, defect species such as H and I centres, which are known to have low thermal stability, are observed suggesting that thermal spike effects do not appear to affect the creation and stabilisation of the defects. At all temperatures of ion bombardment the F/V centre ratio in K 0.8Rb 0.2I is much smaller than that in the pure iodides KI and RbI. Nonetheless the halogen interstitial centres as studied by annealing and subsequent Raman measurements appear relatively similar. The results indicate a modified defect production mechanism in K 0.8Rb 0.2I under heavy ion bombardment; in support of this conclusion X-irradiation of the mixed crystal at 100 and 200 K showed conventional behaviour.