In situ dependence of the formation and behavior of 16 United States Environmental Protection Agency (U.S. EPA) priority polycyclic aromatic hydrocarbons (PAHs) upon the temperature was examined during the experiment carried out on combustion temperatures between 930 and 998 °C in the “Nikola Tesla B” power plant, located 50 km from the Serbian capital, Belgrade. Samples of used lignite and lignite combustion byproducts were collected on a regular basis, and PAHs in collected samples were extracted using ultrasonic extraction with methylene chloride and analyzed by gas chromatography with mass selective detection (GC/MSD). Degradation and formation of PAH molecules were established. Different PAH synthesis mechanisms were noticed by correlation of obtained data, and the presence of phenyl addition/cyclization (PAC) and hydrogen abstraction/acetylene addition (HACA) mechanisms was confirmed. It was concluded that the increase of the combustion temperature has an influence on the increase of total PAH concentrations in combustion byproducts up to the threshold value, while at the same time, there are different distributions of different PAHs in ash and slag. The optimal combustion temperature, producing a minimal PAH concentration in combustion products, for Kolubara basin lignite in the “Nikola Tesla B” power plant is 930 °C.