Van Deemter et al. (1956) were the first to demonstrate that for linear adsorption isotherms, the two-phase adsorption model equation can be replaced by one equivalent equation. The main feature of this equation is a generalized dispersion term which includes the simulation of zone spreading by mass transfer. It is shown in the present paper that equivalence between one- and two-phase models is obtained also for nonlinear Langmuir isotherms if linear driving force conditions prevail and intraparticle diffusion is rate controlling.