A heterogeneous interstitial nucleation model for the radiation-inducedF-coloring in alkali halides is here presented. It applies to the temperature region whereHcenters are mobile, although comparison with ex periments is primarily restricted to the behavior near room temperature. The model assumes the creation ofFandHpairs as the primary event, as well as secondary thermally-activated interstitial processes. These include recombination withFcenters as well as capture by empty traps (very likely impurities) and by all clusters formed after successive interstitial trapping. Therefore, a heterogeneous nucleation model is assumed for the interstitial-aggregation. As the key point of the model it is proposed that one of the interstitial centers (that containing two trapped interstitials) formed in the process of clustering is highly unstable.