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Abstract: The optimum conditions were selected for the chromatographic separation of model mixtures of C12–C40 n-alkanes. For one of the samples of hydrothermal deposits, the extraction conditions of the hydrocarbons were studied and a sample preparation procedure was selected. A procedure is proposed to determine the n-alkanes in samples of hydrothermal deposits by means of gas chromatography-mass spectrometry (GC-MS). The detection limit for the n-alkanes amounted to 3 × 10−9 to 10−8% depending on the components. On the basis of the procedure proposed, the composition of the n-alkanes was studied in the samples of hydrothermal deposits collected at the Mid-Atlantic Ridge (the Broken Spur, Lost City, and Rainbow fields). The analysis performed showed that the samples treated contained C14-C35 n-alkanes. The concentrations of the n-alkanes considered were rather low and varied from 0.002 to 0.038 μg/g. Hypotheses concerning the genesis of the n-alkanes identified were formed.