The Posidonia Shale in the Hi Is syncline in North-West Germany represents a natural maturity sequence from 0.48 to 1.4596 vitrinite reflectance (R-0). In this study, the molecular composition of the acidic compounds of six Posidonia Shale samples with a different maturation level has been determined using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) combined with electrospray ionization (ESI) in the negative ion mode. The changes in the distribution of elemental and compound classes as well as of double-bond equivalents (DBE) and carbon numbers at the different maturity levels are described in detail. Aromatic N-1, N1O1, and N1S1 compounds are preferably formed during ongoing maturation, while the amount of O-2 compounds decreases. With increasing thermal stress, condensation and aromatization of the acidic NSO compounds increases. In the sample of highest maturity (R-0 1.45%), which has been shown to have a relevant shale gas potential, a significant enrichment of nonalkylated and methylated highly aromatic N-1, N1O1, and N1S1, compounds has been observed, which can be the result of thermally induced side chain cracking of compounds with a higher alkylation degree. The other cracking products would be short chain hydrocarbons contributing to a significant amount to the overall gas potential of the Posidonia Shale.