Paylene and benzo[a]pyrene concentration changes during the gradual increase of thermal maturity have been analyzed for the Palaeogene Podhale flysh deposits and other Palaeozoic to Cenozoic sedimentary rock samples. Perylene is present in high relative concentration in the samples of maturation below 0.6% of the vitrinite reflectance (R-r), while in the range of 0.6%-0.7% R-r its abundance rapidly decreased. In the case of samples with vitrinite reflectance higher than 0.7% R-r, perylene compound disappeared completely. Benzo[a]pyrene is also thermally unstable at elevated temperatures and its relative concentration is very low above vitrinite reflectance values of ca. 0.9%. Such results could have important palaeoenvironmental implications. If these five-ring polycyclic aromatic hydrocarbon compounds are characterized by low to moderate thermal stability, then their use as a wood-degrading fungi tracer in the case of perylene and palaeo-wildfire indicator in the case of benzo[a]pyrene is limited only to samples of maturation below the oil window range and below the major phase of oil generation, respectively. These results explain the scarcity of perylene in Palaeozoic sedimentary rocks, which are generally of higher maturation than the younger deposits. (C) 2014 Elsevier B.V. All rights reserved.