Salt marsh sedimentary organic matter (SOM) is a mixture of organic carbon from several sources difficult to identify quantitatively. Geochemical analyses of sediment cores at 4 sites in salt marshes at North Inlet, South Carolina (USA), dominated by Spartina alterniflora, were used to investigate accumulation and diagenesis of organic matter in sediments. Stable carbon isotope ratios ( 6 C ) and concentrations of organic carbon in the fine fraction of SOM ranged from -22 to -17 %O and 2 to 9 %, respectively. 6° values were significantly more positive in sediments from a short-form Spartina zone than from intermediate or tall-form Spartina areas. Samples from the site dominated by short-form S. alterniflora also contained significantly higher amounts of organic carbon than sites closer to the tidal creek, and demonstrated a positive correlation between organic carbon content and isotopically more positive 6° values. Spartina litter buried for 1.25 yr and Spartina lignin had 613C values of -15.35 and -16.34 %o respectively and were significantly more depleted in "C than fresh S. alterniflora (-13.63 %o), but not as depleted as the fine fraction of SOM. However, litter harvested from the marsh surface after 1.25 yr of decomposition had a S1^C value of -13.75 %o. The S^C values of SOM appear to be influenced by a combination of processes, including the selective preservation of isotopically hght refractory carbon, aerobic and anaerobic decay processes, sedimentation of allochthonous carbon from plankton or terrestrial sources, and bioturbation.