Mass transfer equations describing the rate of adsorption for rectangular equilibrium on biporous solids were solved numerically by finite difference technique. Values of parameters affecting the rate of adsorption were varied over wide ranges. It was observed that at a given instant, the microspheres constituting a single biporous solid particle could be classified into three distinct categories : (a) outermost shell of microspheres which are completely (or almost completely) saturated, (b) the middle adsorption shell of microspheres, where adsorption is taking place, and (c) the inner core of microspheres which is yet to be exposed to the adsorbate. A dimensionless parameter beta has been defined. The thickness of the adsorption shell goes on reducing as the value of beta increases. It was found that at large values of beta rate of adsorption is much higher than that predicted by macropore diffusion control. An alternate rate model, incorporating accumulation in adsorption shell, is proposed. Experimental observations of kinetics of adsorption on biporous spherical absorbent particles (molecular sieves and macroporous ion exchange resins) have been analysed. The results indicate that more than one model can correlate data leading to different interpretations.