Hydrogen gas has been successfully produced from sewage sludge by supercritical water gasification. Therefore, whether toxic compounds are synthesized in the supercritical water gasification of sewage sludge should be clarified because of the complex pollutants in sludge. In the current study, the content and forms of polycyclic aromatic hydrocarbons (PAHs) in the solid residue from the supercritical water gasification of wet sludge was investigated using a high-pressure autoclave. The process parameters, such as reaction temperature (400-455 degrees C), reaction time (0-60 min), dry matter content (5.6-23.8 wt%) in the feedstock, and heating period, were varied to investigate their effects on the PAH content and forms. The results showed that PAHs are generated during supercritical water gasification and that high reaction temperature, long reaction time, and low dry matter content favor the formation of PAHs in the solid residue, mainly involving 4-ring PAHs. Moreover, the amount of total PAHs in the solid residue did not exceed the allowable limits of soil quality assessment for exhibition (B class) in China and meets the Canadian soil quality for commercial land use. In addition, the mechanism of PAR formation during supercritical water gasification was discussed in this paper. Crown Copyright (c) 2012 Published by Elsevier Ltd. All rights reserved.