A simple, rapid and sensitive spectrophotometric method has been developed for the determination of dissolved oxygen (DO) in river water samples. The method is based on the reaction of dissolved oxygen with manganous sulphate in alkaline iodide-azide solution and the liberation of iodine by manganese dioxide. The liberated iodine bleaches the violet colour of azure B which is measured at 644 nm. The decrease in absorbance is directly proportional to dissolved oxygen concentration and obeys Beer's law in the range 0.2-1.4 mu gmL(-1). The molar absorptivity, Sandell's sensitivity, detection limit and quantitation limit of the method were found to be 5.86 x 10(4) Lmol(-1) cm(-1), 3.66 x 10(-3) mu g/cm(2), 0.030 and 0.090 mu gmL(-1), respectively.