High-resolution mass spectrometry indicates that the isotopic abundance of (50)vanadium (V) of the Late Cretaceous La Luna petroleum asphaltenes and related source kerogens of marine origin (both highly enriched with V > 2000 ppm) is higher by about 3.5% than that of inorganic source (VOSO4.5H(2)O, Merck). Similar results are obtained with the isotopic analysis of the asphaltenes (containing high V) extracted from the floating asphalts (Dead Sea, Israel). We propose that the difference in the V-50/V-51 values between the La Luna petroleum asphaltenes/source kerogens and inorganic source can be best ascribed to the biological processing of the seawater V. The fact that the isotopic composition of V of the vary over a very narrow range (2.46-2.52) suggests an essentially same (or similar) and fixed (micro)biological source of V. Isotopic analysis was also extended to the methanol-soluble fractions of the La Luna asphaltic petroleums (DM-119/-120/-124) highly enriched with extractable (alkyl) vanadyl-porphyrins (VO2+-P). This analysis shows that the isotopic abundance of V-50 for the methanol-soluble fractions agrees (within the limits of experimental error) with those of the asphaltenes/kerogens. (C) 2002 Elsevier Science Ltd. All rights reserved.