Using the gas chromatographic-mass spectrometric method, the content of 28 carboxylic acids was determined in the thalli of Fucus vesiculosus, palmitic, oxalic and malic acids predominating. In the thalli of Padina pavonica, the content of 27 carboxylic acids was determined. Palmitic, oxalic and oleic acids prevail.Key words: brown algae Fucus vesiculosus Padina pavonica, carboxylic acids gas chromatography mass spectrometry.