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This paper reports the analyses of unusual oils that accumulate in the Uzon Caldera, situated in the central volcanic region of Kamchatka, Russia. Gas chromatography–mass spectrometry (GC–MS) was used to determine the primary constituents, and the 13 C and 14 C compositions provided information about the potential source and age of the oils. The 14 C ages determined are 1030 ± 40 a BP (measured) or 940 ± 40 a BP (conventional). The δ 13 C value is −30.6‰ versus the PDB standard, a value consistent with a biological origin. The nearly contemporary age of the C content indicates a geologically recent origin from biogenic detritus and not by synthesis from mantle C. The biogenic origin is supported by the presence of sterane and hopane biomarkers and the δ 13 C value of the bulk oil. The overall compositions of the oils indicate that they are derived from rapid hydrothermal alteration of algal/bacterial mat detritus buried by volcanic ashfall deposits of the Uzon Caldera. The oils represent the youngest hydrothermal petroleum reported to date.