The organic structures in sapropels sampled from two cores obtained at known locations beneath the southern Black Sea have been characterised. Fluorescence petrography shows the sapropels to occur as layers of impure alginite, similar to 50 mu m thick, within Unit 2 of the sediments. Solid state C-13 NMR indicates the bulk chemical structures to be very similar to those in an immature Type 1 kerogen (lamosite) oil shale with an aromaticity of similar to 0.2. Consistent with the immaturity of the sapropels, which are between 3000 and 7000 years old, temperature programmed reduction showed aliphatic and aromatic sulphides to be the major organic sulphur forms. Alkanes formed from phytoplankton lipids, alkyl benzenes, alkyl naphthalenes and some phenols dominated the mix of volatile compounds identified by pyrolysis-gas chromatography/mass spectrometry. About half of the sapropels remained as an involatile, tarry residue after pyrolysis. The structure of the sapropels is consistent with their formation resulting from marine phytoplankton with only small terrigeneous inputs. Future catagenesis may be expected to decarboxylate the lipids, increase the aromaticity and to dry and compress the muds to form a source rock. (C) 2000 Elsevier Science Ltd. All rights reserved.