Photochemical degradation appears to occur more rapidly in the case of diquat than in the case of paraquat. The light absorption maximum for diquat occurs at a longer wavelength (310 nm) than for paraquat (256 nm), and this partly explains the high rate of photochemical decomposition in the case of diquat. The major degradation products have been identified; they appear to be of low oral toxicity for rats and seem unlikely to produce adverse environmental effects (Black et al., 1966)