This Technical Report (TR) describes a methodology to firstly identify the specific nature of oils spilled in marine, estuarine and aquatic environments and secondly compare the chemical composition of spilled oil or oily samples with that of suspected sources. Specifically, the TR describes the detailed analytical methods and data processing specifications for identifying the specific nature of waterborne oil spills and establishing their correlation to suspected sources. Even when samples or data from suspected sources are not available for comparison, establishing the specific nature (e.g., refined petroleum, crude oil, waste oil, etc.) of the spilled oil may still help constrain the possible source(s) of the spilled oil. This methodology is restricted to petroleum and petroleum products containing a significant proportion of hydrocarbon-components with a boiling point above 200°C. Examples are: crude oils, higher boiling condensates, diesel oils, residual bunker or heavy fuel oils, lubricants, and mixtures of bilge and sludge samples. While the specific analytical methods may not be appropriate for lower boiling oils (e.g. kerosenes, jet fuels, or gasoline), the general concepts described in this methodology, i.e., statistical comparison of weathering-resistant diagnostic ratios, may have applicability in spills involving lower boiling oils. This method is not directly intended for oil spills impacting groundwater, vegetation, wildlife/tissues, soils, or sediments, and although its application in these matrices is not precluded, it requires caution. The reason for caution is that the extractable compounds in these matrices may alter and/or contribute additional compounds compared to the source sample, which if left unrecognised, can lead to “false non-matches”. Including these “non-oil” matrices in this oil spill identification method may require additional sample preparation (e.g. clean-up) in the laboratory prior to analysis and consideration of the extent to which the matrix may affect the correlation achieved. Evaluating the possible effects in these matrices is beyond the scope of this guideline. Whether the method can be used for this kind of “non-oil” matrices may depend on the oil concentration compared to the “matrix concentration” of the samples. In “non-oil” matrices containing a relative high concentration of oil, a positive match can still be concluded. In “non-oil” matrices containing a relative low concentration of spilled oil, a non-match or an inconclusive match could be achieved due to matrix effects.