The absorption spectra and decay lifetimes of theMcenter triplet state (3M) andRcenter quartet state (4R) measured in irradiated KCl are similar to previous results in additively colored crystals. The wavelength dependence of the light‐induced decay indicates that the3Mcenter is deactivated by illumination in its absorption bands at 505, 590, and 685 m&mgr; (85°K), and pumped by illumination in theM1band and also in theFband andKband regions. The initial yield of3Mcenters induced by pulse illumination is proportional to theMcenter density, both for crystals of different initialFcenter density bleached to maximumMband and for a crystal that was bleached through the growth and decline of theMband. The temperature dependence of the dark decay follows a configuration coordinate model in which a transition of 45‐sec half‐time competes with a thermally‐activated step of 0.064 eV activation energy. The 14‐sec dark decay half‐time of the4Rcenter absorption at 710 m&mgr; was found to be independent of temperature from 85° to 200°K. Possible mechanisms of metastable center formation and decay are discussed.