The cis-trans isomerization of chemically activated 1-methylallyl is investigated using RRKM/Master Equation methods for a range of pressures and temperatures. This system is a prototype for a large range of allylic radicals formed from highly exothermic (similar to 35 kcal/mol) OH + alkene reactions. Energies, vibrational frequencies, anharmonic constants, and the torsional potential of the methyl group are computed with density functional theory for both isomers and the transition state connecting them: Chemically activated radicals are found to undergo. rapid cis-trans isomerization leading to stabilization of significant amounts of both isomers. In addition, the thermal rate constant for trans -> cis isomerization of 1-methylallyl is computed to be high enough to dominate reaction with O-2 in 10 atm of air at 700 K, so models of the chemistry of the (more abundant and more commonly studied), frans-alkenes may need to be modified to include the cis isomers of the corresponding allylic radicals. Addition of molecular oxygen to 1-methylallyl radical can forth 2-butene-1-peroxy radical (CH3CH=CHCH2OO center dot), and quantum chemistry is used to thoroughly explore the possible unimolecular reactions of the cis and trans isomers of this radical. The cis isomer of the 2-butene-1-peroxy radical has the lowest barrier (via 1,6 H-shift) to further reaction, but this barrier appears to be too high to compete With loss of O-2.