A novel method of electrochemical pre-separation of Co(II) before detection by chemiluminescence is reported together with the associated instrumentation. The Co(II) ions were selectively pre-separated on a mercury film electrode (MFE) by on-line reduction, then the accumulated metal was oxidised and selectively stripped back into the flowing solution as Co(II). These secondary ions were quantified as a result of their catalytic activity on the chemiluminescent reaction between luminol and hydrogen peroxide that was also induced on-line. The whole sequence was carried out in an automated flow-through system, in which the electrochemical pre-separation of metals was performed in either continuous flow or flow injection analysis (FIA) regimes. The scope of the method, both in terms of selectivity and sensitivity, has been demonstrated and the quantitative determination of Co(II) by the proposed method has been investigated. For a period of continuous flow pre-separation of 4 min, the calibration curve for Co(II) was linear up to a concentration of 100 micrograms l-1, the relative standard deviation was 4% at the 20 micrograms l-1 level and the limit of detection was 0.5 microgram l-1 (at the 3 sigma level). The method was applied to the determination of the cobalt content in a high purity iron sample.