Cyanide ion present in seawater after scrubbing blast furnace and coke ovens gases can be removed by sedimentation of hexacyanoferrate complexes followed by oxidation of residual cyanide with Caro's acid. Zinc ion is removed at the same time by adsorption on the hexacyanoferrate/hydrous ferric oxide precipitate. Sulphide is precipitated as ferrous sulphide, then oxidised by atmospheric oxygen. At 25°C and using an Fe/CN ratio of 1·00, initial concentrations of 50 mg l −1 of CN − and 10 mg l −1 of Zn 2+ in seawater are reduced to 5–7 mg l −1 and 0·1 mg l −1. Subsequent treatment with H 2 SO 5/ CN = 1·2 reduces the [CN −] to 0·1 mg l −1. Treatment of a combined blast furnace/coke ovens effluent ([ CN −] = 24 mg l −1, [ Zn 2+] = 4·0 mg l −1) with Fe/ CN = 1·5 reduced [CN −] to 0·2 mg l −1 and [Zn 2+] to <0·1 mg l −1. Subsequent treatment with H 2 SO 5/ CN = 2·0 reduced [CN −] to 0·2 mg l −1. The process operates best in the pH range 7–9 and so is not affected by the buffer characteristics of seawater.