ABSTRACTAlthough the fibrous plant material called Affenhaar from the middle Eocene lignites of Geiseltal in Germany has been repeatedly studied for over 172 years, modern imaging and chemical analyses have enabled a deeper understanding of its chemical composition and preservation within the parent plant. Known in English as “monkeyhair,” the fibrous material represents a very rare case of fossil laticifers, the latex-bearing ducts of plants. Here we use high-resolution X-ray microcomputed tomography (micro-CT) for the first time to elucidate the anatomy of tissues and their differential degradation within the monkeyhair tree. Even in large, fairly intact stem axes, the center cylinder of secondary xylem inside the trunks or branches is completely absent, presumably due to diagenetic degradation. Nevertheless, there is a moderately well-preserved outer layer of bark and mats of excellently preserved laticifers in the extraxylary zone beneath the bark. Previous chemical analyses revealed that the latex in these laticifers was dominated by natural rubber (cis-1,4-polyisoprene), but also included various triterpenoids. These fossil laticifers also contained large amounts of organic sulfur, suggesting the rubber may have been naturally vulcanized. Pyrolysis GC/MS is also applied to the fossil laticifers for the first time to identify the organic sulfur constituents of this ancient latex. The chemical analysis yields 12 major sulfur compounds, mostly thiophenes, consistent with vulcanized rubber. The vulcanization of these laticifers most likely occurred during early diagenesis, before the degradation of the wood, at low temperatures, and may have been facilitated with other compounds that served as accelerators, activators, and fillers.