Abstract <p>This paper deals with the results of studying the seasonal dynamics of organochlorine pesticides (OCP) in river water with consideration for the physicochemical properties of individual compounds. It is shown that seasonal variability of concentrations for some OCP groups is caused by different mechanisms of physicochemical processes occurring in an aquatic system. It has been established that the content of more hydrophobic and poorly soluble compounds from the dichlorodiphenyl trichloroethane (DDT) group depends predominantly on sorption and photochemical degradation processes, whereas the distribution of less hydrophobic and more soluble hexachlorocyclohexane (HCH) isomers is determined by the equilibrium between the dissolved, sorbed, and gaseous phases. Among all the organochlorine pesticides, particular attention is paid to δ-HCH demonstrating increased mobility and transportability over long distances in a water medium due to its unique physicochemical properties (hydrophobicity constant, distribution coefficient logarithm log <i>K</i>_OW, high solubility in water, moderate volatility, and low Henry constant). The mechanisms of photoisomerization and dechlorination with participation of humic and fulvic acids (humic acids), which may influence on the formation of DDT transformation products, are proposed. The obtained results are of practical importance for environmental risk assessment, OCP migration prediction, and the development of aquatic ecosystem monitoring strategies.</p>