We consider in this paper the mechanism of mass transfer accompanied by an irreversible first-order chemical reaction between two fluid phases, where both diffusion boundary layers at the interface are taken into account. Using the two-film theory and the Danckwerts’ distribution function, a mathematical model for describing this chemical mass transfer mechanism is developed based on the assumption that the concentrations at the interface obey Henry’s law. This new model includes the steady two-film model for chemical mass transfer and for physical mass transfer as special cases