The mechanism of bleaching reaction has not been established in detail as yet. In the present study, bleaching of silver plate was examined with the use of bleach and bleach-fix solution. In the bleach-fixing, the rate was held approximately constant over a period of processing, while, in the bleaching, it was progressively lowered with the time of process. Consequently, it appears that bleaching reaction is inhibited by the formation of silver halide layer on the surface silver plate. It is suggested that migration of electrons or silver ions through the layer account for the ratedetermining step in the process. It can be estimated that since bleaching reaction itself proceeds rapidly, it is not relevant to overall bleach processing speed, unless massive silver is formed in the development.