Zinc oxide is an efficient photosensitiser for the oxidation of Methyl Orange (4-[p-(dimethylamino-)phenylazo]benzene sulphonic acid, sodium salt) in aerobic solutions. Dye oxidation is accompanied by the reduction of oxygen to hydrogen peroxide. The first stage in the reaction involves demethylation of Methyl Orange, followed by cleavage of the azo bond to give colourless products. The dye adsorbs on ZnO, so that extremely low dye concentrations can be rapidly destroyed and the maximum rate of dye oxidation occurs for Methyl Orange concentrations as low as 2.0 × 10^–5 mol dm^–3. This maximum rate is found when < 4% of the ZnO surface is covered by dye. Under these conditions the formal quantum efficiency was 0.5% at 320 nm. The onset of the photochemical reaction corresponded to 395 nm (3.14 eV). Competition studies with radical scavengers show that the oxidising reactants are probably adsorbed hydroxyl radicals and these are formed close to the sites where the dye adsorbs, or migrate rapidly to that region. When Br^– ions are used as radical scavengers the rate of dye oxidation drops to a constant limiting value which is one third of the rate when no Br^– is present. This effect is not observed with the other scavengers and it is attributed to the formation of BrO^–, which can in turn oxidise Methyl Orange.