Secondary propene ozonide and isobutene ozonide were formed in the gas-phase ozonolysis of ethene with added acetaldehyde and acetone, respectively. Combined with the formation of hydroperoxymethyl formate and methoxymethyl hydroperoxide in the ethene- ozone reaction system in the presence of HCOOH and CH3OH, respectively, formation of the secondary ozonides reveals a close similarity between the gas-phase and the liquid- ...