Ultra-trace amounts of cobalt in sea-water were determined by flow injection on-line sorbent extraction–preconcentration coupled with electrothermal atomic absorption spectrometry. With sodium diethyldithiocarbamate as the complexing agent and C₁₈-bonded silica reversed-phase sorbent as the column material, a 42-fold enhancement of peak area, compared with that for direct 40 µl sample introduction, was obtained with a 120 s preconcentration and 5.6 ml of sample. The atomization signal was directly proportional to the preconcentration time. An enhancement factor of 210 and a detection limit of 1.7 ng l^–1 could be achieved with a 10 min sample loading time (28 ml). Hence, the extremely low cobalt concentration of 4.0 ng l^–1 in National Research Council of Canada certified reference material NASS-1 Open Ocean Seawater could be determined within 12 min. Results obtained for sea-water and estuarine water certified reference materials agreed with the certified values with a relative standard deviation of 5.0%.