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a b s t r a c tIn order to compare the effects of weathering on two types of mixed oil, simulated weathering experiments were performed. The first sample was a mixture of two fresh oils and the second sample was a mixture of one fresh oil and another oil sample that had undergone a serious weathering process. Comparative studies evaluated decay rates and changes in diagnostic ratios of some fingerprinting biomarkers. Results showed that the mixing process affected the weathering rate of some compounds in the oils and also that certain diagnostic ratios are more suitable for estimating mixing proportions. A Pr/n-C 17 versus Ph/n-C 18 plot can be used to identify the end-numbers of mixed oils and the (C 13 + C 14 )/(C 25 + C 26 ) ratio is a useful diagnostic ratio to detect the degree of weathering. Finally, hopane, sterane, and alkylated PAH fingerprints were found to give useful insights about the sources of the mixed oils.