Three conformers of the isopropyl-substituted Criegee intermediate, (CH3)(2) CHCHOO, have been observed by Fourier transform microwave spectroscopy. The transient species was produced using a pulsed electric discharge of a gas mixture of 1,1-diiodo-2-methylpropane/O-2 diluted in Ar or Ne. The use of different carrier gases in the supersonic expansion reveals the difference of the collisional relaxation process between anti-conformers. The conformational relaxation pathways have been investigated theoretically and are presented as well. In light of these results, the previous study on the ethyl-substituted Criegee intermediate, where the absence of one of the four possible conformers was associated with collisional relaxation processes, has been re-examined. Here we report the detection of a new conformer of the ethyl-substituted Criegee intermediate observed using Ne as the seeding gas. Published by AIP Publishing.