The recent controversy over the accuracy of the higher dissolved organic carbon (DOC) concentrations reported by Sugimura and Suzuki (1988; Marine Chemistry 24, 105-131) obtained using high-temperature catalytic oxidation (HTCO) methods has been further critically examined during inter-calibration exercises between HTCO and ultra-violet (UV) persulphate DOC analyzers. The accuracy of these DOC methods is currently still unclear. This study and other work indicates that the HTCO method of DOC analysis may measure 5 to 60% more DOC than UV persulphate methods. An increase in DOC oxidation efficiency (6 to 8%) was achieved by adding low concentrations of Hg salts (0.004 M in the actual sample; Hg(NO₃)₂ and HgCl₂) to the sample stream in a UV persulphate DOC analyzer. This increase in DOC oxidation efficiency implies that there is a fraction of DOC that is not measured by the UV persulphate method of DOC analysis, when used without the addition of Hg salts. The increase of DOC concentrations approaches the higher levels of DOC measured by the HTCO method. To investigate the size fractionated distributions of organic carbon in the estuarine system a two year field study was undertaken in the Beaulieu estuary during 1990 to 1992; the Beaulieu river system is characterized by an organic rich drainage basin and is situated in the southern part of Britain. Ultrafiltration methods were developed for isolating colloidal organic carbon (COC) fractions from natural waters using the unique properties of Anodisc Anopore filters. Sources of organic carbon were differentiated using δ13C signatures and background parameters were also measured to aid interpretation. Colloidal organic carbon represents a large fraction of carbon in the estuarine waters; up to 3.6 x 'true' DOC (TDOC; i.e. the DOC concentration after filtration through Anodisc Anopore 0.02 μm filters) and up to 12 x POC concentrations.