Leather production results in large volumes of waste water containing high levels of sodium sulphide, which has to be treated before disposal. The use of ozone to facilitate this oxidation was considered by first studying ozone-sodium sulphide reaction. The sodium sulphide solutions were found to be adequate models for the tannery effluent. The reaction order was found to be second order overall (first order in both sulphide and ozone). The true reaction order was found to be 0.33 for sulphide in the initial stages of the reaction. The use of ozone in conjunction with either UV radiation or a manganese catalyst was found to have no benefit to the rate of sulphide oxidation. The reaction products were thiosulphate, sulphite and sulphate. A mathematical model based on a first order consecutive reaction scheme was developed. The liquid phase mass transfer coefficient for physical absotption of ozone into water was estimated to be k'La=4.76 x 10-3 S-I, while the overall transfer coefficient during the ozone-sulphide reaction was estimated to be Kt.a=().0719 S-I. This gave an enhancement factor of 15.1 which indicates that ozone decomposition reactions may be negligable. It was estimated that a full scale ozone system would require the application of 120 kg of ozone per hour for 12 hours to treat 200 Itt waste water. An economic comparison of this proposed system indicates that operating costs for ozone would be approximately 6 times greater per m3 waste water than the costs for the current aeration treatment