The qualitative and quantitative composition of water and suspended matter hydrocarbons was assessed. The transformation of these compounds during their migration in the «water-suspension» system of the river delta in a tropical climate was studied on the example of one of the deepest branches of the Mekong River (Ham Luong River) in Vietnam. The material for the study was samples of surface and bottom water taken along the riverbed in November-December 2022. The physicochemical parameters of the environment were measured using a multimeter in situ. The qualitative and quantitative composition of hydrocarbons in the water and suspended matter was determined by gas chromatography on the basis of the Scientific and Practical Center for Spectrometry and Chromatography of the FRC IBSS. Biogeochemical markers of the origin of hydrocarbons were used to identify probable sources of organic matter. Individual physicochemical indicators of the aquatic environment (pH, O₂, salinity, temperature, concentration of suspended matter) were characteristic for the rivers of the lower Mekong River and were within the limits characterizing the favorable state of the reservoir during the wet season. The content of hydrocarbons in the water of the Ham Luong River averaged 0.061 ± 0.019 mg·L⁻¹. These indicators were quite high, exceeding the sanitary standards (0.05 mg·L⁻¹) for fishery reservoirs, or approaching this value. The concentration of hydrocarbons in suspended matter averaged 0.019 ± 0.009 mg·L⁻¹. An increase in the content of hydrocarbons in the suspended phase was noted in the area where the river flows into the sea, in comparison with the sections of the river located upstream. Hydrocarbons in the water were of mixed origin, and contained both biogenic components of autochthonous and allochthonous origin, and traces of oil pollution. In suspended matter, along with biogenic compounds, there are also biodegraded petroleum compounds. Organic compounds, both of allochthonous and petroleum origin, coming from the catchment areas of the Ham Luong River, which is especially pronounced during the wet season, as well as from the surface of the river, further undergone biotransformation during the transition to a suspended state. As a result, the composition of n-alkanes in suspended matter differed significantly from that in water samples.