Four conformers of the doubly substituted methyl-ethyl Criegee intermediate, C2H5C(CH3)OO, have been observed by Fourier-transform microwave spectroscopy. The transient species was produced using a pulsed electric discharge of a gas mixture of 2,2-diiodobutane/O2. The conformational preferences differ from those observed previously for related alkyl-substituted Criegee intermediates. The observation of small splittings in the spectra due to the internal rotation of only one methyl group enabled us to determine the barrier heights of the hindered methyl rotation for the four conformers, which have been compared with those reported for other methyl-substituted Criegee intermediates.