The process of competitive diffusion-adsorption of polymers of differing chain lengths in porous and non-porous adsorbents is modelled mathematically. The dynamic events are shown to be dictated by several competing factors linked to diffusion, adsorption, disengagement, etc. A striking observation of an intermediate plateau before the final equilibrium in the adsorption curve is attained, is predicted by the model and validated with some very recent experimental data. Some important implications of the work on development of process for polymer fractionation on the basis of differential adsorption based on difference in chain lengths is discussed.