The fundamental absorption spectra of CuCl nanocrystals in glass samples are investigated in the energy range 3 4 eV at a temperature of 300 K with the aim of elucidating the kinetics of the initial stage of the formation of the CuCl phase in the glass. The CuCl phase is grown in the glass under stepwise annealing at temperatures of 500, 615, and 707°C. The kinetics of variation in the intensity and the shift of the maximum in the exciton absorption spectra of CuCl nanocrystals are studied in the course of annealing of the glass samples. It is established that, for all the temperatures under investigation, the formation of the CuCl phase begins with the transient stage that involves the fluctuation formation of supercritical nuclei of the CuCl nanomelt. At a temperature of 500°C, the transient stage gives way to the stage of a rapid increase in the number of supercritical nuclei of the CuCl phase. At temperatures of 615 and 707°C, the transient stage gives way to the stage of an intensive growth of nuclei without a considerable increase in their number. The number of nuclei formed during the transient stage at 707°C is smaller than that observed after the transient stage at 500°C by a factor of 24. However, the sizes of the nuclei formed at 707°C are larger than those observed after the transient stage at 500°C. This difference is explained by the fact that the diffusion length of Cu+ ions controlling the formation of the CuCl phase increases with increasing temperature. The experimental data on the kinetics of the formation of the new phase in the glass are in good agreement with the Zel’dovich-Frenkel classical theory of the formation of a new phase, which accounts for the stage of the formation of critical nuclei.