Gasification is regarded as a prospective and promising method for rendering sewage sludge harmless. Sewage sludge, apart from energetically desirable compounds, is the source of toxic and hazardous organic and inorganic contaminants. The tests were aimed at assessing transport and transformations of organic and inorganic contaminants during sewage sludge gasification in the sludge-gasification-solid and liquid waste by-products system. The chemical qualitative analysis of the dried sewage sludge and waste by-products produced during gasification covered the concentrations of major elements, mineral elements, phosphorus, alkaline metals and selected heavy metals. The sewage sludge samples, as well as solid and liquid gasification waste by-products, were qualitatively and quantitatively analyzed using Gas Chromatography-Mass Spectrometry to assess their contamination with low-molecular organic compounds. Gasification promotes the migration of certain inorganic compounds, e.g., phosphorus or zinc from the sewage sludge to the solid phase formed after the sludge was treated thermally.