Abstract: An isothermal one-dimensional reactor of infinite length is considered where two co-current phases flow with different rates also containing a solute. Interfacial reaction between the solutes, as well as axial diffusion with different diffusitivities are assumed to take place. Defining the overall flux as the response in such a system, the impulse response is, as a rule, equal to the density of the residence time distribution. A straightforward algorithm to determine the asymptotic semi-invariants of the impulse response for large values of the axial coordinate is given.