A general applicable model has been developed which can predict mass and heat transfer fluxes through a vapour/gas-liquid interface in case a reversible chemical reaction with associated heat effect takes place in the liquid phase. In this model the Maxwell-Stefan theory has been used to describe the transport of mass and heat. The description of the transfer rates has been based on the film model in which a well-mixed bulk and a stagnant zone are thought to exist. In this paper results obtained from the Maxwell-Stefan theory have been compared with the results obtained from the classical theory due to Fick. This has been done for isothermal absorption of a pure gas A in a solvent containing a reactive component B. Component A is allowed to react by a unimolecular chemical reaction or by a bimolecular chemical reaction with B to produce component C. Since the Maxwell-Stefan theory leads to implicit expressions for the absorption rates, approximate explicit expressions have been derived. In case of absorption with chemical reaction it turned out that the mass transfer rate could be formulated as the product of the mass flux for physical absorption and an enhancement factor. This enhancement factor possesses the same functional dependency in case Fick's law is used to describe the mass transfer process. The model which has been developed in this work is quite general and can be used for a rather general class of gas-liquid and vapour-liquid transfer processes. In this paper (Part I) only isothermal simulations will be reported to show the important features of the model for describing mass transfer with chemical reaction. In many processes such as distillation, reactive distillation and some absorption processes, heat effects may play an important additional role. In Part II non-isothermal processes will be studied to investigate the influence of heat effects on mass transfer rates.