The paper presents the study of the absorption mechanism of organic pollutants such as formaldehyde and oxalic acid by natural zeolite and peat materials modified by iron (III) ions using the limited volume method.The relevance of this paper is a search for the use of inexpensive and available natural sorbents for waste water purification from ecologically hazardous water-soluble organic pollutants. The impact produced by the solution concentration and temperature and the size of natural composite particles on the sorption rate of organic substances is described in this paper. The effective factors for diffusion and energy of the process activation are calculated. It is found that the mixed-diffusion kinetics is the determinant stage of the process. In case of peat and zeolite composites, the internal diffusion prevails in the mixed-diffusion kinetics.