Effects of growth process and after growth thermal treatment on daylight coloration and afterglow of CsI:Tl scintillator have been studied. Induced absorption, photoluminescence, thermo-luminescence and afterglow were used as tools for the identification/analysis of crystal defects. Formation of color centers and their dependence on growth process were established. A possible diffusion mechanism of color centers during the heat treatment of the crystal is proposed. Results of our experiments show that a post growth annealing minimize those number of color centers that are directly responsible for afterglow and daylight coloration. It has been demonstrated that excess of centers associated with Tl+ dimers are also responsible for the large afterglow in this crystal. © 2010 American Institute of Physics.