A more precise criterion for the formation of a steady-state sorption front is the convergence of the integrals which characterize the quantity of a sorbate in the leading edge of the front and the unused capacity of a sorbent behind the front. Existence of an asymptotic solution of the equation of sorption dynamics is not sufficient for convergence of these integrals, if the line passing through the points of the equilibrium isotherm, which correspond to the initial concentratons in the solution … and in the sorbent, is tangent to the isotherm at one of these points.