Pyrolysates of the organic carbon-rich and oil-prone rocks of the Kimmeridge Clay Formation (KCF) are dominated by a sulfur-rich unresolved complex mixture (UCM). Structural characterization of this UCM by preparative capillary gas chromatography, gas chromatography (GC), gas chromatography/mass spectrometry (GC/MS), isotope-ratio-monitoring GC/MS, elemental analysis, nuclear magnetic resonance spectroscopy, and Fourier transform infrared absorption, as well as chemical degradation by desulfarization and ruthenium tetroxide treatment, revealed that this UCM probably consists of a large number of sulfur-bound and oxygen-bound, short-chain carbon skeletons. These skeletons are part of oligomeric structures of diverse molecular weight that elute during GC analysis as an UCM. These skeletons most likely originate from carbohydrates that have been incorporated into macromolecular organic matter through sulfurization during early diagenesis. Upon pyrolysis, this macromolecular material is transferred into the oligomeric structures present in the UCM. Sulfur-rich UCMs have also been identified in several other kerogens, suggesting that carbohydrate sulfurization could be an important mechanism for the preservation of sedimentary organic carbon. In addition, it is likely that the sulfurized carbohydrates form a substantial part of sulfur-rich kerogens and that these thermally labile structures cause the early generation of petroleum from sulfur-rich kerogens.