Batch kinetic experiments were performed for adsorption of nitrobenzene and benzonitrile from aqueous solution onto activated carbon, the isotherms of which were only partially expressed by Freundlich equations. The experiments were carried out with adsorbents which had been equilibrated with a certain concentration of the solution in advance, to obtain adsorption rates representing certain ranges of the amount adsorbed. The intraparticle mass transfer rate was confirmed to be dominated by surface diffusion through the determination of pore diffusivity using a non-adsorptive substance, while the external mass transfer resistance could not be neglected. The surface diffusivities thus obtained were analyzed from the viewpoint of the non-ideality of the driving factor of surface diffusion, the amount adsorbed. Introduction of mobility of adsorbed molecule, derived through a treatment similar to ordinary diffusion in liquid phase, and assumption of the change of mobility with the energy of adsorption well explained the dependence of surface diffusivity in the whole range of concentration investigated.