Safe and ecological treatment of sewage sludge eg. as a fertilizer, requires information about their contamination. This also applies to by-products (ashes) formed during the gasification of the dried sewage sludge. The aim of this study was to present infrared spectroscopy (IR) as a way of assessing the presence of contaminants in the dried sewage sludge and ashes. Two types of sewage sludge was used in the study. The sludge was coming from wastewater treatment plant operating in mechanical-biological system and sewage sludge taken from the mechanical-biological-chemical wastewater treatment plant. Ash formed during the fixed bed gasification process of sewage sludge was also analyzed. The results of the analysis were compared to results obtained using the most popular method of analysis - gas chromatography coupled with mass spectrometry (GC-MS). IR analysis showed significant differences in the degree of contamination of analyzed dried sewage sludge and ashes. There also have been note significant differences in the degree of contamination between compared dried sewage It was also determined that, the ashes from gasification were not contaminated by all of the organic substances originally identified in the dried sewage sludge. Similar conclusions were drawn based on the GC-MS analysis. The results lead to the conclusion that the infrared spectroscopy method can be used as a preliminary assessment of the degree of contamination of the samples prior to chromatographic analysis, eg. in screening studies.