This contribution describes the development and some highlights of the internationally agreed standard procedure CEN/TR 15522–2:2012: “Oil spill identification – Waterborne petroleum and petroleum products – Part 2: Analytical methodology and interpretation of results based on GC-FID and GC-MS low resolution analyses” [1]. In particular, handling of changes caused by weathering of spilled oil is described here: PW plots (partial weathering plots) allow a proper and unequivocal identification of oil, despite those changes. CEN/TR 15522–2:2012 has been produced by Bonn-OSINet (Oil Spill Identification Network of experts within the Bonn Agreement). Researchers from all over the world have cooperated and contributed to its development. This method has been continuously improved and tested over the last decade. Cooperation of laboratories culminated in COSIweb (Computerized Oil Spill Identification), an online program which includes a huge database of more than 2,200 oil samples at the time of writing and an automatic evaluation system. This web-based resource provides the possibility to handle raw data produced anywhere in the world and to evaluate these data as if they were produced in a user’s own laboratory.