Batch kinetic studies of the adsorption of water on 4A molecular sieve cylindrical pellets from its solution in isopropyl alcohol, reported earlier, have been mathematically modeled. Each cylindrical pellet was assumed to be composed of microspheres (zeolite crystallites) of equal size. Partial differential equations describing mass transfer in a biporous cylindrical pellet were written and solved numerically to obtain the concentration of adsorbate in the macropores, fractional saturation of individual microspheres, and fractional saturation of the entire pellet. Fractional saturation of microspheres at any instant involves three different regions of the adsorbent pellet : 1) an outer saturated or semisaturated zone, 2) a middle adsorption shell where most of the adsorption is taking place at a given instant. and 3) an unexposed core. The thickness of the adsorption shell decreases as the rate of diffusion (a) increases in the microspheres and (b) decreases in the macropores. This physical picture led to the development of a simpler approximate model which incorporates accumulation in the adsorption shell. This has been named the adsorption shell model. This model is amenable to a hand calculator and could be employed to correlate batch kinetic data.