Abstract A direct comparison is presented here between dissolved organic carbon (DOC) analyses done with the historical routine wet chemical oxidation method (persulfate oxidation), the high temperature catalytic oxidation method of Sugimura and Suzuki (MRI instrument), and a commercial high temperature combustion analyzer (Shimadzu instrument). The sample set used for the comparison included stations in a tidal urban river, the salinity gradient of an estuary, and continental shelf and slope waters, including those to a depth of 2000 m. Both high temperature combustion methods measured considerably more DOC (1.5 to 3.5 times) in the coastal waters than did the conventional persulfate method. In the fresh water and estuary, the commercial instrument measured only slightly higher values than did the conventional method, while the MRI instrument measured almost twice the concentration. Throughout all the environments, the MRI instrument measured about twice the concentration measured by the commercial instrument. An inverse linear correlation can be demonstrated between DOC and apparent oxygen utilization with data from each of the methods, but it is argued that this does not necessarily imply ‘oceanographic consistency’.