The autoxidation of alkylperoxy radicals (RO2, where R is organic) is an important degradation pathway for organic compounds in a wide range of chemical systems including Earth's atmosphere. It is thought to proceed by internal H-shift reactions and, for unsaturated radicals, cyclization. However, experimental data on specific reactions steps for unsaturated RO2 is scarce. This work investigates the unimolecular reactions of 1-butenyl-O2, 1-pentenyl-O2, 1-hexenyl-O2, and 2-methyl-2-pentenyl-O2 radicals near room temperature (302 ± 3 K) experimentally, by monitoring the radicals directly, and theoretically. The experimental rate coefficients are in good agreement with those determined with high-level quantum calculations, confirming that cyclization can be competitive with H-shift in some cases. However, the products observed experimentally with two different mass spectrometers suggest that all the peroxy radicals studied lead to fast decomposition (k > 1 s−1) after the isomerization step. While the mechanisms for these decompositions could not be fully elucidated theoretically, they question whether these channels contribute to propagation or to termination of the autoxidation chains.