Ozonolysis is an important removal pathway of alkenes in Earth’s troposphere, generating highly reactive carbonyl oxide compounds (R1R2C=O+–O–) known as Criegee intermediates. Unimolecular reactions of Criegee intermediates can contribute to non-photolytic generation of hydroxyl radical (OH), a key oxidant in the troposphere. Their bimolecular reactions with water vapor, SO2, organic acids, and other atmospherically relevant species are related to the formation of secondary organic aerosols, which can impact the air quality and climate. Laboratory direct detection of Criegee intermediates was achieved through an alternative synthetic route involving reactions of photolytically generated monoiodo-alkyl (or -alkenyl) radicals with O2. Criegee intermediates with various substituents, including isopropyl-substituted Criegee intermediate, methyl-ethyl-substituted Criegee intermediate (MECI), 2-butenal oxide, and 3-penten-2-one oxide, were characterized on their first π* ← π transition using a ground state depletion method under jet-cooled conditions at ultraviolet (UV) and visible wavelengths. In addition, the second π* ← π transition band of methyl vinyl ketone oxide (MVK-oxide) was characterized at shorter UV wavelengths. Electronic excitation of Criegee intermediates leads to O–O bond breakage, releasing O-atoms and carbonyl coproducts. The photodissociation dynamics was examined using velocity map imaging with selective detection of O (1D) products. A reduced impulsive model was developed to simulate energy partitioning among photofragments, which yielded good agreement with experimental total kinetic energy release (TKER) distributions. Bimodal TKER distributions were observed for photodissociation of CH2OO and MVK-oxide at short wavelengths, which arise from different dissociation pathways involving multiple singlet states and conical intersections. The unimolecular decay of MECI was investigated using multiplexed photoionization mass spectrometry under thermalized conditions (298 K, 10 Torr), revealing the formation of hydroxybutanones via OH roaming with a 7% yield. The theoretical prediction was a 10% yield for roaming products (from anti-MECI) under atmospherically relevant conditions (298 K, 760 Torr), competing with OH radical products. Complementary experiments under limited collision conditions detected characteristic CH2OH fragments of roaming products for various syn-methyl-substituted Criegee intermediates and demonstrated that OH roaming is common in the unimolecular decay of Criegee intermediates.