Ozonolysis of isoprene, one of the most abundant volatile organic compounds emitted into the Earth?s atmosphere, generates two four-carbon unsaturated Criegee intermediates, methyl vinyl ketone oxide (MVK-oxide) and methacrolein oxide (MACR-oxide). The extended conjugation between the vinyl substituent and carbonyl oxide groups of these Criegee intermediates facilitates rapid electrocyclic ring closures that form five-membered cyclic peroxides, known as dioxoles. This study reports the first experimental evidence of this novel decay pathway, which is predicted to be the dominant atmospheric sink for specific conformational forms of MVK-oxide (anti) and MACR-oxide (syn) with the vinyl substituent adjacent to the terminal O atom. The resulting dioxoles are predicted to undergo rapid unimolecular decay to oxygenated hydrocarbon radical products, including acetyl, vinoxy, formyl, and 2-methylvinoxy radicals. In the presence of O2, these radicals rapidly react to form peroxy radicals (ROO), which quickly decay via carbon-centered radical intermediates (QOOH) to stable carbonyl products that were identified in this work. The carbonyl products were detected under thermal conditions (298 K, 10 Torr He) using multiplexed photoionization mass spectrometry (MPIMS). The main products (and associated relative abundances) originating from unimolecular decay of anti-MVK-oxide and subsequent reaction with O2 are formaldehyde (88 � 5%), ketene (9 � 1%), and glyoxal (3 � 1%). Those identified from the unimolecular decay of syn-MACR-oxide and subsequent reaction with O2 are acetaldehyde (37 � 7%), vinyl alcohol (9 � 1%), methylketene (2 � 1%), and acrolein (52 � 5%). In addition to the stable carbonyl products, the secondary peroxy chemistry also generates OH or HO2 radical coproducts.