AbstractPhotochemistry of the herbicide 3,5‐dibromo‐4‐hydroxybenzonitrile (bromoxynil) (I) was investigated by irradiating at approximately 313 nm. Aqueous phase photoreactions of 0.078–7.800 × 10−5m solutions of I were carried out at different pH values. Quantum yields for the loss of I in buffered solutions were 0.008 (±0.0004), 0.048 (±0.0024), and 0.044 (±0.0022) at pH 2.6, 7.0, and 11.0, respectively. In neutral and basic conditions, I absorbed more strongly at wavelengths>290 nm, an environmentally significant region. Phototransformation of I was monitored by HPLC and UV‐VIS spectrometry. All photoreactions of I gave rise to the generation of two products, 3‐bromo‐4‐hydroxybenzonitrile and 4‐hydroxybenzonitrile. The former photoproduct was tentatively identified from its mass spectral data. The photoproducts can be accounted for with a proposed mechanism