Abstract A rapid, sensitive and reproducible flow-injection (FI) analysis method is described for the spectrophotometric determination of cobalt(II) using 4-benzylpiperidinedithiocarbamate as a chromogenic reagent. The method involves the measurement of the absorbance of the colored cobalt(II)-(4-benzylpiperidinedithiocarbamate) 2 complex at 640 nm. In the system, a microcolumn (MC) containing strong cation-exchange resin was placed between injection manifold and spectrophotometer to increase interactions between cobalt(II) and reagent, and for preconcentration of cobalt(II). The linear range of the proposed FI method was 0.6–100 μg l −1, and the limit of detection (signal/noise=3) was 0.1 μg l −1 for a 10 μl injection of cobalt(II) ion solution at a sampling rate of 60 h −1. The relative standard deviation (R.S.D.) for 0.6–100 μg l −1 cobalt(II) was <1.0% ( n=5). The precision and accuracy of the method were checked by analysis of certified reference metals, and the method was applied to river, sea and hot spring water samples.