Alterations in the composition of a Light Arabian crude oil has been studied in situ during 12 months in an artificially contaminated site. After 12 months of experimentation, C17/Pr and Cts/Ph ratios could not be used, because the isoprenoYds were no longer stable. In contrast, a high degree of stability of biomarkers, especially hopanes, was observed. These are very useful tools to assess alterations in other less stable classes of saturated compounds (n-alkanes and isoprendids) contained in petroleum. Marked degradation of both IIalkanes and isoprenords (using 17a21p C30 hopane as "internal standard") has been evidenced. The use of biomarkers also made it possible to observe a burying process which occurred down to 6 cm deep after 12 months of ~cxperimentation.