Abstract The F-coloring efficiencies of NaCl:Pb++ and NaCl:Sr++ have been measured during stage I as a function of lead and strontium precipitation into the metastable and stable phases, the optical absorption spectrum of the doped crystals was analyzed after each annealing treatment. On the other hand, strontium precipitation was monitored using Eu++ as an optical probe taking advantage of the fact that this ion can be incorporated into the Sr-precipitates producing emission bands characteristic of each type of precipitate in which it is embedded. The results indicate that the metastable and stable second phase precipitates of these ions contribute in a quite different manner to the coloring processes.