In this study the fate of oil in marine sediment inhabited by the polychaete Nereis diversicolor is investigated. Data from two preliminary biodegradation experiments have been analysed using univariate plots as well as multivariate data analysis (Principal Component Analysis). Results show that when Nereis diversicolor is not present in the sediment the degradation of oil is very slow and weathering processes, e.g. evaporation and dissolution, are the only processes changing the composition of oil. When Nereis diversicolor is present in the sediment marked changes in the oil composition occur as indicated by changes in several biodegradation indices. The results indicate that multivariate data analysis is potentially a strong tool in the evaluation of changes in oil composition due to weathering and biodegradation processes, and that the isomeric patterns of C1-Fluorenes could be applied as indices for infaunal degradation. The present work is considered a pilot study indicating the need for further investigations of interactions between bioturbating infaunal organisms and persistent sediment pollutants.