The mechanism for the formation of M centers from F centers upon irradiation in the F band is discussed. The evidence favoring F2+ (or M+) centers as intermediate states is shown to be less convincing than formerly believed. Measurements of the activation energy of the reaction rate have been made for F-to-M photoconversion in additively colored KCl (0.35 eV), KBr (0.39 eV), and CsBr (∼0.1 eV). The "activation volumes" for F-to-M conversion in KCl and CsBr have been measured. They are much smaller than those estimated for free anion diffusion. The same mechanism for F-to-M conversion seems to obtain in these three salts.