The nature of sedimentary organic matter from mangroves and the continental margin of eastern Brazil (8 degrees-24 degrees S) has been investigated in order to obtain information on sources and diagenetic processes. The organic matter content of mangrove sediments is three to four times higher than the maximum content of continental margin sediments. Downslope distribution of organic carbon, nitrogen, amino acids and hexosamines shows an enrichment in water depths between 800 m and 1000 m. The distribution of individual amino acids and hexosamines is highly variable in sedimentary organic matter from mangroves and the continental margin, and the observed compositional differences are mainly due to diagenetic alteration. Organic matter is highly reactive in mangrove sediments and refractory in slope sediments, which suggests that recycled organic matter forms the bulk of the organic matter in upper slope sediments. From the qualitative differences between organic matter in mangrove sediments and in continental margin sediments it appeals that mangrove-derived organic matter is not a significant component of the latter. It is also conceivable that organic matter is retained in mangroves or that it undergoes severe decomposition in coastal waters.