The doctoral thesis deals with the issue of weathering and determination of fire accelerators in samples. The main goal of the doctoral thesis is to design and verify a progressive forensic method of sample processing to determine fire accelerants in fire investigation. One of the sub-goals of the work is to determine changes in the composition of selected gasoline residues. Model samples of matrices of cotton carpet and ignitable liquid - gasoline (fire accelerator) representing samples obtained from the fire scene under different processing conditions (primarily temperature and time) were examined. Gasoline EVO 95 Slovnaft was used as a fire accelerator, and cotton carpet ARYA 05 BEIGE/BLUE as a matrix. In laboratory conditions, the samples were exposed to different weathering intervals (0 minutes - 240 hours). Three variants of the experiments were performed: unburned samples poured with gasoline (USL), burned samples with the presence of gasoline (BSL), and burned samples with gasoline extinguished with water (ESL). Residual gasoline compounds in carpet samples were determined by gas phase extraction (HS) coupled with gas chromatography (GC) and mass spectrometry (MS). In the research, the following selected gasoline compounds were closely compared: toluene, ethylbenzene, p-xylene, 1,2,3-trimethylbenzene, 1,3,5-trimethylbenzene, and naphthalene. The most significant changes were determined in reducing the intensity of most volatile compounds (alkyl alkanes), where toluene decreased by 11.92% in the unburned USL sample. A similar but less pronounced trend occurred in the burned BSL samples, where toluene decreased by 7.40%. More minor changes occurred in alkyl derivatives of benzene, which include most of the 15 gasoline markers, e.g. (1,2,3-trimethylbenzene, 1,3,5-trimethylbenzene). Conversely, the heavier compound, naphthalene showed an increase in the intensity of 0.42%. In the case of burned samples, the increase was 1.31%. The most significant changes can be observed after four or five hours, indicating the need for rapid sampling and subsequent timely analysis. The work results show that the ESL wet carpet fiber sampling method is the most effective and reliable, as almost constant proportions of representation were determined for each compound. It was also confirmed that the delayed collection and analysis of samples in an accredited laboratory, even after ten days, provide equally reliable analysis results when proving the presence or, conversely, the absence of ignitable liquid at the fire scene.