In this series of investigations with a sulfur-sensitized cubic silver bromide emulsion with 9-methylthiacarbocyanine dye as a spectral sensitizer, we propose a new mechanism of dye desensitization according to which the enhancement of dispersion of image centers by cyanine dyes brings about a decrease in sensitivity. In the light of the fact that introduction of crystal defects into emulsion grains depressed the dispersion of image centers, a study was made of the dependence of the desensitization on the crystal defects introduced into the emulsion grains without causing any change in their size, shape, and halogen composition, effected by adding a solution of the dye during their growth process. The observation that the crystal defects actually depressed the desensitization strongly supports the proposed mechanism of dye desensitization.