An automated instrument is described which measures the oxygen demand of the organic content of a fluid based on oxidation by hydrogen peroxide with UV light. Residual hydrogen peroxide remaining after oxidation of the organic compounds is determined by gas liberated from H2O2 decomposition by catalase, measured by a low flow gas meter. Oxygen produced by the internal chain reaction of H2O2 with hydroxyl free radicals is also measured and since the amount of peroxide added is known, the amount of peroxide used to oxidize the organic compounds can be calculated. A linear relationship between organic strength (measured as COD or BOD) and the output of the instrument was obtained for dilutions of raw sewage (25-150 mg l-1 COD and undetectable to 54 mg l-1 BOD), a synthetic sewage (100-1400 mg l-1 COD and 68-952 mg l-1 BOD) and effluents from a creamery (50-600 mgl-1 COD) and potato processing factory (50-350 mg l-1 COD). The instrument is fully automated in the present version giving one sample measurement every 55 min and could be used on-line as the basis of a control strategy for wastewater treatment plants. The instrument is robust, with relatively low capital cost and cost of ownership.