Unimolecular isomerization and decomposition reactions of alkylperoxy (RO2), hydroperoxyalkyl (QOOH), and hydroperoxyalkylperoxy (O(2)QOOH) radicals play important roles in the low-temperature oxidation of hydrocarbons. In this study, these reactions have been investigated by the CBS-QB3 quantum chemical method, and the variation of the rate parameters by the structural change of alkyl groups has been studied systematically for the rule-based construction, of the low-temperature oxidation mechanisms of arbitrary noncyclic alkalies. The results can be well-interpreted in terms of the group additivity and the ring-strain effect of the cyclic transition states. To extract the important processes needed for the chemical kinetic modeling, the competing reaction channels were compared in detail by steady-state analysis with the high-pressure limiting rate constants. The importance of some reactions of O(2)QOOH radicals, which have not been considered in the previous modeling studies, such as the hydrogen exchange reactions between, -OOH and -OO center dot groups and hydrogen shift reactions from non-OOH sites, is suggested.