ABSTRACTIn the search for the mechanisms involved in the immobilization of organic nitrogen in humified remains of vascular plants, the efforts of the present investigation were directed toward the examination of the transformation of nitrogenous compounds during the peat and coal stage by means of solid‐state nuclear magnetic resonance (NMR) spectroscopy. While accumulation of heteroaromatic‐N is not detected in most of the studied peat layers, a clear shoulder in the chemical shift region of pyrrole‐ or indole‐N is observed in the solid‐state 15N NMR spectrum of material from the deepest (and thus oldest) peat layer underlying the sapropel from Mangrove Lake, Bermuda (10000 years). This points to the assumption that transformation of nitrogen occurs between an advanced stage of peatification and an early stage of coalification. The observed sudden alteration in nitrogen functionality indicates that continuous accumulation of newly synthesized or selectively preserved biogenic structures is not responsible for the presence of heteroaromatic‐N in these fossilized deposits. It seems rather likely that abiotic conditions, occurring during advanced sediment maturation, have an effect on the observed N transformation. With increasing coalification, pyrrole‐type‐N becomes the dominant form in the macromolecular coal network. Pyridine‐type‐N was only detected in a coal of anthracite rank.