INVESTIGATIONS carried out here with the selective herbicide ioxynil (3,5-diiodo-4-hydroxybenzonitrile)1 have shown its mode of action to be complex. The compound inhibits the Hill reaction in isolated chloroplasts, strongly uncouples oxidative phosphorylation, inhibits photo-synthetic phosphorylation and when exposed to ultraviolet light there is evidence that free radicals are produced with the loss of iodine from the moleculo2,3. In further investigations of the breakdown of ioxynil in soil and plant we have found that hydrolysis to 3,5-diiodo-4-hydroxybenzoic acid occurs with the amide as an intermediate product. The liberation of iodine in these systems has also been demonstrated.