Data are presented for particulate organic carbon (POC) and particulate nitrogen (PN) concentrations in the Humber Estuary and tidal River Ouse Estuary. The POC data were derived from approximately monthly surveys and are consistent with data reported for suspended particulate matter (SPM) in the non-tidal River Ouse (the freshwater river) and with SPM, or bed sediments, in estuarine ecosystems such as the Mississippi, Delaware, San Francisco Bay, Tolo Harbour, the Vellar Estuary and Cochin Backwater, as well as the Loire, Gironde, Ems and Tamar Estuaries. Relative to the dry weight of SPM, the Humber-averaged organic carbon and nitrogen percentages during the year February 1995-March 1996 were 2.6 +/- 0.6% (mean and S.D.) and 0.21 +/- 0.04%, respectively. The ratio of Humber-averaged POC to Humber-averaged PN was 13 +/- 3. Higher POC levels were observed near the Humber's mouth and in the adjacent coastal zone during 'bloom' conditions, and in the upper estuarine reaches during large, winter and springtime freshwater inflows. At these times of high runoff, the POC content of SPM increased progressively up-estuary from the coastal zone to the tidal River Ouse. When inflows became very low, during late spring to early autumn of 1995, both the freshwater-saltwater interface (FSI) and the strengthening turbidity maximum (TM) moved further up-estuary and the POC content of SPM in the upper reaches of the Ouse became lower compared with that immediately down-estuary. This led to a poorly defined POC maximum near the confluence of the Humber, Ouse and Trent, before POC eventually decreased again towards the coastal zone. The lower POC contents in the upper estuarine reaches of the tidal Ouse may have been partly due to POC respiration by heterotrophic bacteria attached to SPM within the TM, consistent with the severe oxygen depletion observed there during high turbidity, summertime spring tides.