A fundamental description of gas-liquid mass transfer with reversible consecutive reaction has been given. The Higbie penetration theory has been used and numerical simulations were carried out for isothermal absorption. Although the model can be adapted to reactions of general stoichiometric and kinetic orders, results have been provided for unit orders only. In Part I, results have been presented for the case of equal diffusivities of all the chemical species involved. In Part II, the effect of unequal diffusivity on the absorption rate and, hence, the overall enhancement factor in consecutive reaction systems has been presented in detail. Results presented here are dedicated to the case where both reaction steps are considered to be reversible. Finally, the model presented in this paper has been used to determine the selectivity towards the intermediate species using data previously reported in literature. The model system of chlorination of p-cresol in 1,2,4-dichlorobenzene has been used for this purpose. The strong influence of the mass transfer parameters on the selectivity of the intermediate has been shown.