Cobalt is quantitatively retained as cobalt-2-nitroso-4-sulfonic acid (nitros-S)-tetradecyldimethylbenzylammonium (TDBA+) ion-pair complex on microcrystalline naphthalene in the pH range 6.2-9.0 from a large volume of its aqueous solution of various complex samples. After filtration, the solid mass consisting of cobalt complex and naphthalene is dissolved in 5 ml of dimethylformamide (DMF) and the metal is determined by flame atomic absorption spectrometry (FAAS). Cobalt complex can alternatively be quantitatively adsorbed on tetradecyldimethylbenzylammonium chloride-naphthalene adsorbent packed in a column and determined similarly. The calibration curve is linear over the concentration range 0.2-11.5 micrograms of cobalt in 5 ml of the final DMF solution. In this case, 0.2 microgram of cobalt may be concentrated in a column from 250 ml of aqueous sample where its concentration is as low as 0.8 ng/ml. Eight replicate determinations of 3.0 micrograms of cobalt in the final DMF solution give a mean absorbance of 0.045 with a relative standard deviation of 1.8%. The sensitivity for 1% absorption is 59 ng/ml. The interference of a large number of anions and cations on the determination of cobalt has been studied and the optimum conditions developed utilized for its trace determination in various alloys and biological samples. The method may also be used for the determination of cobalt in some environmental samples.