Two classes of unimolecular reactions of peroxy radicals are key to autoignition, namely, intramolecular H-atom shift (which promotes autoignition) and concerted HO2 elimination (which inhibits autoignition). Olefin groups are prominent functional groups in biodiesel fuels. This paper explores the effects of the presence of an olefin group and its position on the kinetics of unimolecular reactions of peroxy radicals. CBS-QB3 calculations were carried out for 10 selected alkyl- and alkenylperoxy radicals. Transition-state theory was used to determine the temperature dependence of the high-pressure limiting rate constants, and Rice-Ramsperger-Kassel-Marcus/master equation simulations were performed to determine the pressure dependence of selected rate constants. Tunneling effects were computed using the asymmetric Eckart potential. The contribution of internal rotors to partition hindered-rotor treatment. functions were included by using the