The diploma thesis deals with the factors influencing the determination of accelerants in fire debris. The aim of the work was to use experimental results to evaluate different methods under different conditions. The work was prepared based on a literature search of several authors dealing with the issue. The next aim was to compare how different materials can affect the quality of a samples. In the experiments, the two most used accelerators were used - gasoline and diesel, and commonly used materials such as wood, carpet, and PVC. The SPME method and the passive headspace method were used as the sample preparation method. It is clear from the results that the sorption properties of the individual materials influence the determination of the accelerators from the samples. This feature can significantly affect the correct identification of the accelerants used. Another factor that can affect the identification of accelerators is time. First and foremost, timely and correct sampling of the fire using cans or plastic bags. One of the reasons for misidentification of the accelerator is the possibility of microbial degradation. The critical point in the analysis process is the selection of a suitable extraction method. It is necessary to choose a suitable extraction time and temperature. The optimal time required to determine the accelerators from the samples appears to be 15 minutes for both accelerants. When detecting accelerants, it is necessary to select a suitable extraction method depending on the sample. The choice of the method of determination is influenced by many factors such as the type of material and its thickness, the possibilities of using the solvent, the time during which the extraction will take place. The structure of the material significantly affects the correct identification of accelerators from the matrix, e.g., more complicated fibrous materials such as wool and silk appear more complicated for the determination of accelerators from the matrix. All these factors significantly affect the work of investigators of the causes of fires.