Organic matter of five lithologically different rocks, belonging to the Grodziec Beds (Upper Namurian A strata), was geochemically characterized by various instrumental methods. Mineral composition of rocks, known from previous experiments, was investigated by X-ray diffraction and organic matter as a whole was characterized by infra-red spectroscopy. Next, extracts of organic matter were analysed by chromatographic methods, especially by gas chromatography-mass spectrometry (GC-MS) analysis applied to investigate aliphatic and aromatic hydrocarbon fractions. It was found that sedimentary organic matter of the Grodziec Beds predominantly comes from terrestrial sources, i.e. higher vascular plants, deposited in aerobic (oxic) environment. It shows features of type III kerogen or type II kerogen on bacterial/terrestrial origin of primary biogenic matter. Thermal maturity was estimated by means of various geochemical parameters based on distribution of biomarkers and alkyl-substituted aromatic hydrocarbons. It was found that the thermal maturity of organic matter of investigated rocks is at the end of diagenesis or beginning of catagenesis. Input of older migrating bitumen was suggested as an explanation for increased values of some thermal maturity parameters in the case of one of sandstones. Water-washing and/or biodegradation has partially changed the composition of organic matter, especially in the case of porous sandstone, where such interactions are possible. Mineral composition of the host rock seems to be important here, protecting organic matter of mudstones against such post-depositional alterations.