We describe a combination of selected ions as a terminating ion which is useful for transient isotachophoresis (ITP) in capillary zone electrophoresis (CZE) for the determination of nitrite and nitrate in seawater. In addition to 150 mm sulfate as the principal terminating ion, 10 mm bromate was added to a sample solution as the additional terminating ion. Artificial seawater containing 3 mm cetyltrimethylammonium chloride (CTAC) was adopted as a background electrolyte (BGE). The limits of detection (LODs) for nitrite and nitrate were 2.2 and 1.0 mug/L (as nitrogen), respectively. The LODs were obtained at a signal to noise ratio (S/N) of 3. The values of the relative standard deviation (RSD) of peak area for these ions were 1.9 and 1.4%. The RSDs of peak height were 1.7 and 1.9%, the RSDs of migration time 0.11%. The proposed method was applied to the determination of nitrite and nitrate in a proposed certified reference material for nutrients in seawater, MOOS-1, distributed by the National Research Council of Canada (NRC). The results almost agreed with the assigned tolerance interval.