The kinetic equations of the formation of the F centers (vacancies) and the V4centers (di-interstitials) in KBr are formulated on the basis of the present knowledge of the defect interactions. In order to explain the experimental observations that the vacancy concentration is proportional to the 0.8th power of the irradiation dose and that the formation rate is independent of the dose rate, a model is invoked in which the di-interstitial formation is enhanced in the vicinity of the di-interstitials previously created. It is shown that the solutions of the kinetic equations explain the experimental growth curves and that the present model can account for the nucleation of the cluster of the di-interstitials suggested by Hobbs.