In this work, an on-line system for preconcentration and determination of cobalt at mug l(-1) level is proposed. Amberlite XAD-2 functionalized with alpha-nitroso-beta-naphthol (NN-XAD) packed in a minicolumn was used as sorbent. Cobalt (II) ions were sorbed in the minicolumn, from which it could be eluted by hydrochloric acid solution. This eluate reacted with Nitroso Salt R and the absorbance of the complex was read at 510 nm in a spectrophotometer. Signals were measured as peak height by using instrument software. Achieved sampling rate was 45 samples per hour. Analytical parameters were evaluated and the results demonstrated that cobalt can be determined, with sample pH in the range of 7.0-8.5, preconcentration time of 60 s with a sample flow rate of 5.0 mL min(-1). The detection limit (3 s) was 6.0 mug l(-1) and the precision (assessed as the relative standard deviation) reached values of 6.1 to 1.9% in cobalt solutions of 10 to 300 mug l(-1) concentration, respectively. The proposed procedure was applied in natural waters. Recoveries of spike additions (10, 25, or 50 mug l(-1)) to natural water samples were quantitative (92-106%).