The Danish maritime oil spill response is a trilateral co-operation between the Admiral Danish Fleet, the Environmental Protection Agency (EPA) and the National Environmental Research Institute (NERI). NERI is responsible for the analysis and identification of oil spills accidentally or illegally discharged at sea as part of the defensible oil spill assessment. In 1999 NERI analysed 418 oil samples from about 50 actual spills and suspected ships. All samples were analysed according to the Nordtest method, NT Chem 001 "Oil Spill Identification", including initial screening by GC/FID and subsequent GC/MS fingerprinting of selected samples for final identification. With this technique more than 500 individual components in every oil are analysed by the recording of 30 selected ions. By comparing recorded fingerprints, including PAHs and biomarkers, differentiation by inherent variations due to source input and generation and influences by environmental weathering can be revealed. The procedure is illustrated by a recent spill in Danish territorial zone where an oil tanker en route from the Baltic Sea to the North Sea was suspected of illegal discharge of waste oil more than once along its route. Analysing GC/MS fingerprints of selected samples from the spills at open sea and affected shores along the route and from the tanker indicated that it was responsible for the spills. As the number of oil samples was relatively large correlations, diagnostic ratios and principal component analysis were applied to facilitate and improve the comparison of analytical data from both the GC/FID screening and GC/MS fingerprinting.