Oil spills, especially in water have happened throughout the years. The consequences from this kind of incident have always been a threat to marine life and take a lot of money and time to clean up. The forensic investigations of oil usually focus on oil spill in water, however, this kind of investigation can be applied on more areas, such as in sexual offense cases and arson. The oil from the crime scene and the oil from the suspected source are analyzed and compared to each other to see if the compositions match. The aim of this thesis was to optimize the clean-up process of heavy fuel oil (HFO) using solid-phase extraction (SPE) according to the European Committee for standardization (CEN) 15522-2 method. HFOs are black oils, containing particles and soot which will decrease injector performance. Thus, a cleanup procedure is attractive when analyzing such samples. The three factors of optimization in this study were elution volumes, solvents, and SPE sorbents.The SPE method used in this study was to use the cartridges as a filter; therefore, the samples were both loaded and eluted with an organic solvent, in this case dichloromethane (DCM) or dichloromethane/heptane (DCM/Hp). The gas chromatography mass spectrometry (GC-MS) analysis method used is described in Annex B in the CEN 15522-2 method.Concluding, increasing the eluent volume did not indicate improvement on the extraction method. In terms of the solvents, both dichloromethane and dichloromethane/heptane showed similar results. The percentage weathering plots (PW-plots) showed that the SPE sorbents (dual layer florisil/Na2SO4, florisil and silica gel sorbents) worked similar. The overall conclusion from this thesis work was that further optimization must be made before implementing the method.