The model of isothermal adsorption in a fixed bed consists of the mass balance equations for the fluid phase and for the solid phase. Taking into account the widely used simplifying assumption that all parameters and variables are constant in a perpendicular to the flow direction, the model consists of partial differential equations with three independent variables: z, r and t. The solution of the model equations requires much computation. The computations can be greatly simplified by using approximations for the intraparticle diffusion rates. One of the methods applied for the simplification of the problem is based on the assumption of the intraparticle concentration profile. The well known linear driving force approximation results from a parabolic intraparticle concentration profile. In this communication an approximation based on a modified exponential profile Eq. 26) has been applied. Such an approximation of the concentration profile satisfies the condition of symmetry of the concentration field. The obtained intraparticle mass transfer rate equation (Eq. 38) reduces at short times to the well known Eq. (41), resulting from the penetration theory. A series of numerical tests has been performed in order to demonstrate the application of the proposed approximation under various operational conditions. Furthermore, in order to check the accuracy of the obtained results, such operational conditions were chosen, for which analytical solutions of the problems are available. The results for the case with negligible external mass transfer resistances (Bi --> infinity) are shown in Fig. 4. As we can see, the results obtained from this approximation are very accurate, particularly in the short time region. The results for the case with finite external mass transfer resistances (Bi < infinity) are shown in Fig. 5. As we can see, the accuracy is satisfactory for all Blot numbers and increases with decreasing Biot number. The last test concerned the case of adsorption from a finite medium in the case of negligible external mass transfer resistances. The results (Fig. 6) were satisfactory. Numerical tests show that the proposed approximation can be applied in the modelling of adsorption systems. Taking into account high accuracy of the approximation in the short time region, it can be particularly useful in the case of fast cyclic adsorption-desorption processes. Furthermore, intraparticle concentration profile proposed in this work can be applied to examinations of the processes in a catalyst pellet.