The mass transfer rate in porous spherical pellet for irreversible equilibrium adsorption systems has been analysed. Both the existence of mass transfer resistances inside and outside the pellet, and the as parallel mechanisms of pore diffusion and surface diffusion, have been considered. The range of divergences appearing in experimental determination of diffusion coefficients for a real non-linear adsorption system, if the approximation of irreversible isotherm is applied, has been examined.