A phenomenon of mass transfer with chemical reaction takes place whenevertwo phases which are not at chemical equilibrium with one anotherare brought into contact. Such phenomena are made up of a number of elementarysteps, which may be summarised as follows.(i) Diffusion of one or more reactants from the bulk of phase I to theinterface between the two phases.(ii) Diffusion of the reactants from the interface towards the bulk ofphase 2.(iii) Chemical reaction within phase 2.(iv) Diffusion of reactants initially present within phase 2, and/or ofreaction products, within phase 2 itself due to concentration gradients whichare set up by the chemical reaction.Steps (ii) , (iii) and (iv) may take place simultaneously, and thus mutuallyinterfere; the overall phenomenon resulting from steps (ii) , (iii) and (iv) takesplace in series with step (i). If step (i) is rate controlling, the overall rate isnot influenced by the chemical reaction, and the process may be regarded as a simple mass transfer phenomenon which is not influenced by the reaction rate.The analysis of mass transfer with chemical reaction is of interest whenthe overall phenomenon resulting from steps (ii), (iii) and (iv) is rate-controlling.Chemical reactions which accompany mass transfer between phases areusually classified as, slow, fast and instantaneous.In this paper the" nature of these reactions is described and the mathematicalmodels related to such phenomena are presented and discussed.