When the process of adsorption on the fixed bed of activated carbon is used to minimize the content of pollutants in a stream, e.g. in the technology of water treatment and renovation, the shape of the initial segment of the break-through curve of the so-called low concentration region is important. Usually, it is assumed that in this region the mass transfer is dominated by external film diffusion. The paper presents a theoretical analysis and experimental verification of this assumption. It was concluded that in the low concentration range, the process was determined by equilibrium parameters of the adsorption system and external mass transfer characterized by the intraparticle diffusion coefficient. Results of experimental investigations of phenol, p-nitrophenol and benzoic acid adsorption from aqueous solutions on a fixed bed of activated carbon AG-5 are presented.