The optical density change of the CuCl-doped borosilicate glasses(2 wt&percent; CuCl+52.5SiO2−40B2O3−7.5Na2O(wt&percent;))heat treated at 550 °C for 0–10 hrs was measured with temperature at spectral vicinity of the CuCl resonant absorption and the effect of size and distribution of CuCl precipitate particles on the optical spectra was investigated. The optical absorption spectrum obtained during heating and cooling cycle showed a typical hysteresis loop because of the melting and solidification of the CuCl particles embedded in the glass matrix. The shape of the optical absorption curve measured at 375 nm with temperature upon melting of CuCl particles was dependent on the size and the distribution of the CuCl particles. While the average size of the CuCl particles increased with heat treatment time, the size difference of the precipitated CuCl particles was found to be decreased with time before 1 h and then increased. The optical absorption spectra of the CuCl doped glass was also predicted theoretically and the predicted spectra curve was found to be in good agreement with experimental data. It is proposed that if the semiconducting particles embedded in an optically transparent matrix and the melting point of semiconductor is lower than that of the matrix material, the average size and size distribution of the particles can be estimated by the optical absorption measurements with temperature. ©1997 American Institute of Physics.