Hydroperoxide decomposition by hindered amine light stabilizers (HALSs) is an important process producing active HALS derivatives and has been studied energetically. The decisive active species of the decomposition, however, has not been proposed yet. In this article, HALS nitrosonium that forms in the decomposition of a hydroperoxide by HALSs and a ring-opening product of the nitrosonium, a nitroso compound, are proposed as active species for hydroperoxide decomposition. Furthermore, the reactivity of the nitrosonium with phenols is discussed. (c) 2006 Wiley Periodicals, Inc.