A study has been made to establish the mechanisms of photochromic processes in vacuum-deposited AgBr films. These films are darkened by UV irradiation and bleached by heating in air. To establish the mechanisms, films heated in vacuo were compared. We found that the darkened films were bleached by heating in air but not in vacuo. The photochromic bleaching mechanism is explained in terms of recombination of dissociated Ag and Br. By heating in air, oxide layers are formed on the surfaces of AgBr films, and the oxides prevent the escape of dissociated Br gas from the films to the atmosphere. Therefore, dissociated Ag and Br species confined in the films together can be recombined upon heating. This recombination makes the photochromic repetition possible. With the films heated in vacuo, however, Br gas is allowed to escape through the surface because of the absence of an oxide barrier, and thus the darkened films cannot be bleached. Schematic models of photochromic processes are proposed.