Abstract: A general rate equation to predict the effect of combined film diffusion, intraparticle diffusion and chemical interaction on the rale of ion exchange reaction in a finite bath is derived. From this general rate equation, previous rate equations on ion exchange by a shell-progressive mechanism may be deduced. The rate of ion exchange is a function of ξ and η. From the parameters ξand η, it is now possible to determine the circumstances prevailing in reactions in which one of these resistances is the rate determining step.