Secondary organic aerosol (SOA) formation from isoprene ozonolysis was investigated using a Teflon bag reactor under dry and humid conditions. Both the number and volume concentrations of SOA were found to be decreased by the addition of water vapour. Gas- and particle-phase product analyses with a negative ion chemical ionization mass spectrometer show that oligomers composed of stabilized Criegee intermediates as the chain units contribute to the SOA formation and that water vapour inhibits stabilized Criegee intermediates from forming the oligomers, resulting in the suppression of SOA formation. Additionally, it is suggested that a portion of stabilized Criegee intermediates other than CH2OO have low reactivity toward H2O, and thus can be involved in the oligomer and SOA formation even under humid conditions. Volatility estimation predicts that the oligomers containing even one or two stabilized Criegee intermediates can be partitioned into the aerosol phase.