[en] The effect of biodegradation of stranded oil from the Baltic Carrier oil spill of March 29, 2001, was studied using multivariate data analysis. Samples of oil smudged stones were collected from the beach at Hestehoved, Denmark and analyzed. The data obtained from gas chromatograph-mass spectrometry was analyzed by three different methods: (1) univariate analysis using the diagnostic biodegradation ratios nC17 per (nC17+Pr) and nC18 per (nC18+Ph), (2) multivariate data analysis of the isomeric pattern of tetramethylnaphthalenes, and (3) a more comprehensive analysis of 21 diagnostic biodegradation ratios selected from among a much larger number of variables through a selection method based on quantitative reliability criteria. There were two separate biodegradation steps, as revealed by the analysis. Initial biodegradation occurred during the first 2.5 months after the spill. For the months of April-May, alkanes, C1-phenanthrenes and C1-fluorenes degraded during the first phase. In samples collected from May to mid August, the degradation of C2-phenanthrenes and C2-dibenzothiophenes was apparent. The authors noted the highest degree of biodegradation in samples collected at 8.5 months. The strength of the multivariate approach was confirmed in this study. The method revealed the changes that occur among highly complex samples. It can also revealed which compounds are affected and the direction of the major changes. 16 refs., 1 tab., 5 figs