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We have already proposed possible mechanisms of hindered amine light stabilizers (HALS), including the step in which HALS works as light stabilizer.This step consists of the regeneration of phenols, by reaction with a hydroxylamine of HALS, from benzoquinones derived from a phenolic antioxidant incorporated as radical scavenger. This paper provides clear evidences of the step, namely: benzoquinones are activated by UV light, resulting in the function as UV absorbers, and then are reduced by HALS, such as parent HALS and hydroxylamine, to form corresponding phenols such as hydroquinones.The reverse reaction of this step is also examined.In conclusion, actions of HALS, in practical use, are discussed on the basis of kinetic observations.

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