Samples of recent sediments and marine organisms from seven coastal lagoons along the Mexican coast of the Gulf of Mexico were analyzed by gas chromatography and GC-MS coupled system to determine the present levels of fossil hydrocarbons. Results show that the highest concentrations of fossil hydrocarbons are present in organisms and sediments located near petrochemical plants and oil refineries, indicating that petroleum hydrocarbons are being released into the coastal lagoons. Stable carbon isotope ratios (12C/13C) of sedimentary organic carbon in sediment and organism samples were also determined. For most of the lagoons, the delta 13C ratios for sediments ranged from -19.0 to -23.9%, anomalous ratios ranging from -26.8 to -29.3% were shown for sediments and organisms (oysters) of two of the studied areas, probably because of the presence of organic carbon from anthropogenic terrestrial sources.