Reflectance versus depth diagrams for vitrinite from Carboniferous beds and for bituminite from the Late Permian Kupferschiefer in northern Germany led to the conclusion that there may exist upper limits for the applicability of reflectance as a maturity parameter at 6% R(r) and 8% R(max) for vitrinite or 4% R(r) and 5% R(max) for bituminite. The vitrinite threshold values may mark the end of the diagenesis (anchimetamorphism) field and the boundary between meta-anthracite and semi-graphite.