The study examined the impact of the process of underground gasification on the petrographic composition of lignite from the Turow deposit. The lignite fed into the reactor is mainly xylodetritic and sapropelic lignite. As a result of the gasification process in the reactor, a cavern and lignite residues were formed. The samples collected from the individual areas were subjected to a detailed petrographic analysis. On the basis of petrographic analyzes it can be concluded that lignite is being transformed as a result of thermal gasification process. Transformation areas, closely related to the temperature prevailing in the reactor, are clearly visible. Moving from the cavern, the following areas can be distinguished: ash area, coke area, degassed area containing dried lignite, cracked transversely to stratification and dried lignite area. These areas can be easily correlated with the distribution of temperature in the reactor. A burned out cavern was formed in the place where the temperature exceeded the ignition temperature of lignite (approx. 500 degrees Celsius). The layers of sapropelic lignite were heavily dried and degassed during the gasification process. Both gelification and fusain layers are not observed within these layers. The ash and portions of coked lignite, characterized by the highest temperature and the largest concentration of oxygen, are mainly observed in the fire channel. The examination has confirmed that a large part of coal was only dried. The biggest changes were observed directly at the cavern and in the fire channel. Sapropelic coal has changed to a lesser extent. The changes include drying and partial degassing. In some places, a thin layer of coke can be observed. The most commonly observed transformations include strong gelification and the formation of fusain. (C) 2017 Elsevier Ltd. All rights reserved.