Criegee intermediates (R₁R₂C=O⁺O⁻) are transient, zwitterionic carbonyl oxides formed during alkene ozonolysis. Their photochemistry and reactivity are strongly subject to the substituents (R₁, R₂) and conformational forms. This work investigates such structural effects through experimental and high-level theoretical approaches. The Criegee intermediates are studied under jet-cooled conditions using 10.5 eV vacuum ultraviolet (VUV) photoionization detection or under thermal conditions using multiplexed photoionization mass spectrometry (MPIMS), which enables simultaneous kinetic and product analyses. The ultraviolet-visible (UV-Vis) spectrum of the methyl-ethyl-substituted Criegee intermediate (MECI) is characterized on the strong π*← π transition via a ground-state depletion method. A large rate coefficient (298 K, 10 Torr) is also obtained for MECI bimolecular reaction with SO₂ leading to formation of SO₃. Systematic comparison between MECI and resonance-stabilized methyl vinyl ketone oxide demonstrates that the extended conjugation red-shifts the UV-Vis absorption and hinders the bimolecular reactivity with SO₂. The prototypical methyl-substituted Criegee intermediate (CH₃CHOO) has syn and anti conformations distinguished by the -CH₃ orientation relative to the terminal oxygen. MPIMS investigation reveals that anti-CH₃CHOO reacts barrierlessly with dimethylamine via a 1,2-insertion mechanism, while syn-CH₃CHOO remains unreactive. Infrared (IR) excitation of jet-cooled CH₃CHOO in the CH stretch overtone region induces rapid unimolecular decay of both conformers. The resultant IR-VUV ion-dip spectrum reveals dominant contribution of the syn-CH₃CHOO due to its greater stability and higher population. Organic hydroperoxides are also studied as key reactive species in atmospheric oxidation cycles. The 282 nm photodissociation dynamics of tert-butyl-, cyclopentyl-, and cyclohexyl-hydroperoxide are investigated using velocity map imaging to obtain the velocity and angular distributions of the OH products. The total kinetic energy release (TKER) distributions are bimodal with low-TKER components indicative of internal conversion to the ground electronic state prior to dissociation. The high-TKER components are consistent with an impulsive model arising from dissociation in the excited electronic state, but exhibit greater internal energy partitioning in the two cyclic systems where low-frequency ring vibrations are efficient energy sinks. Complementary theory provides insights into the isotropic angular distributions observed for the three systems.