The results of a study of the chemical composition of aquatic plants are presented: Stratiotes aloides L. (S. aloides) and Ceratophyllum demersum L. (C. demersum), which intensively accumulate biomass in the reservoirs of the Kaliningrad region and do not have useful applications. It was found that the crude biomass of S. aloides and C. demersum contains 92.66±1.5% and 91.51±1.5% moisture, 1.23±0.28% and 1.46±0.28% protein, 1.8±0.1% and 3.4±0.2% fiber, 1.55±0.1% and 2.13±0.2% ash, respectively. The lipid content is low in the biomass of S. aloides (0.1%) and is less than 0.1% in the biomass of C. demersum, while the fatty acid composition of lipids of both plant species is characterized by a predominance of saturated and monounsaturated fatty acids. Of the studied macronutrients, the highest content of potassium (44.14±3.06 mg/kg) was found in the biomass of S. aloides, calcium (27.72±3.06 mg/kg) and sodium (23.58±3.06 mg/kg) in the biomass of C. demersum. Vitamins of group B, vitamins A, C, carotenoids and chlorophyll (qualitatively) were identified in the composition of the studied aquatic plants. Vitamin E was also found in the biomass of S. aloides. The mass fraction of all identified vitamins was 0.04% of the crude biomass. Plants are sources of polyphenolic compounds, their content was 1.21±0.05 and 1.5±0.05 mg of gallic acid equivalent/ ml in extracts of S. aloides and C. demersum, respectively. The content of polyphenolic compounds, vitamins A, C, E, carotenoids, and chlorophyll allows the macrophytes considered to be potential sources of biologically active compounds in the food industry. The study of the content of the main contaminants (toxic elements, pesticides, polychlorinated biphenyls, nitrosamines) in the biomass of S. aloides and C. demersum showed compliance of the obtained indicators with the requirements of the technical regulations of the Customs Union.