A chromatographic-mass spectrometric study of the low molecular weight metabolome (LMWM) of water smartweed (Persicaria amphibia (L.) Delarbre, family Polygonaceae Juss.), growing in various biotopes of Lake Ladoga, was carried out for the first time during the flowering phase in order to reveal its qualitative and quantitative composition. Essential oil containing low molecular weight organic compounds (LMWOC) from dried plants was obtained by Clevenger hydrodistillation with steam. The composition and content of LMWOC in the essential oil were detected using a TRACE DSQ II chromato-mass spectrometric complex (Thermo Electron Corporation) in a programmed temperature mode using decafluorobenzophenone and benzophenone as internal standards. A total of 124 LMWOC were detected, of which 110 were identified. Carboxylic acids (50-60 % by content), aldehydes (9-14 %) and hydrocarbons (6-9 %) prevailed among the LMWOC in the water smartweed. The LMWM of P. amphibia from the biotope exposed until recently to heavy hu