The properties of pyrolysis and combustion of five tars, which are produced from the gasification of five sewage sludges in the external heated downdraft reactor under the gasification temperature of 800 °C and the air flow rate of 0.05 m3/h, were studied by thermal gravimetric analysis with a heating rate of 10 °C/min in the atmosphere of nitrogen and air, respectively. The results indicate that the thermal processes of five tars in the two atmospheres can be divided into two stages. The releasing of light organic matters happens in the first stage, and there also exists the decomposition reaction during pyrolysis. The second stage is the decomposition or combustion stage of the remaining heavy organic matters for the pyrolysis or combustion. The reaction mechanisms in the two atmospheres have been studied with Coats-Redfern equation. It shows that the mechanisms of five tars in the same atmosphere are different.