AbstractThe photooxidation of 2,6‐di‐tert‐butyl‐4‐methylphenol (I) and 2,2′‐methylene‐bis(4‐methyl‐6‐tert‐butylphenol) (II) sensitized with methylene blue in dichloromethane was investigated. The oxidation of phenol I proceeds at a reduced rate, but otherwise similar to the oxidation in methanol, that is, with the formation of 2,6‐di‐tert‐butyl‐4‐methyl‐4‐hydroperoxy‐2,5‐cyclohexadiene‐1‐one (III, R = tert‐butyl). On the other hand, bisphenol II remains almost unoxidized in dichloromethane. This is due to the formation of stilbenequinoid derivatives of type VI which slow down the oxidation even at insignificant concentrations. The effect approaches that of β‐carotene. Phenol I has a much weaker tendency towards the formation of an analogous derivatives, i.e. 3,5,3′,5′‐tetra‐tert‐butylstil‐benequinone (VIII), which however is also an active retarder of oxidation. A more detailed study carried out with VIII indicates that the mechanism of its effect is the same as for β‐carotene, that is, the quenching of singlet oxygen. Stilbenequinone VIII used in a fourfold concentration has almost the same effect as β‐carotene, but unlike the latter it is much more stable towards oxidation. The act