In the atmosphere, the gas-phase reactions of ozone with biogenic and anthropogenic alkenes are significant sources of hydroxyl (OH) radicals, which are often termed the atmosphere’s detergent. Alkene ozonolysis proceeds through energized carbonyl oxide species, known as Criegee intermediates, that can undergo unimolecular decay to OH radical products. Jet-cooled stabilized Criegee intermediates are investigated to obtain their unimolecular decay dynamics upon ultraviolet (UV) and infrared (IR) excitation in a collision-free environment. The UV dissociation dynamics of CH2OO and syn-CH3CHOO are investigated using the velocity map imaging technique to characterize the velocity and angular distributions of the oxygen-atom products. IR activation of prototypical alky-substituted Criegee intermediates (syn-CH3CHOO, (CH3)2COO and CH3CH2CHOO) is used to drive hydrogen atom transfer from the methyl group (or -CH2-) to the terminal oxygen, followed by unimolecular decay to OH radicals, which are detected by UV laser-induced fluorescence. The OH appearance time profiles are obtained by varying the IR pump – UV probe time delay. Unimolecular decay rates are measured upon vibrationally activation of syn-CH3CHOO, (CH3)2COO and CH3CH2CHOO in the vicinity of the transition state (TS) barriers. In addition, the unimolecular decay of syn-CH3CHOO and (CH3)2COO to OH products is shown to occur at energies significantly below the TS barriers. Initiation of the hydrogen transfer process at these lower energies provides a stringent test of quantum mechanical tunneling effects in the unimolecular decay rates. The measured experimental rates for syn-CH3CHOO and (CH3)2COO in this deep tunneling regime are approximately 100 times slower than those in the vicinity of the TS barriers. The experimental results are in very good accord with statistical Rice-Ramsperger-Kassel-Marcus calculations of the microcanonical decay rates with tunneling through the TS barrier. Thermal unimolecular decay rates for alkyl-substituted Criegee intermediates are predicted from energy-dependent microcanonical decay rates, and are found to have significant contribution from energies that are much below the TS barrier.