Methyl vinyl ketone oxide (MVKO) is an important Criegee intermediate in the ozonolysis of isoprene. MVKO is resonance stabilized by its allyl moiety, but no spectral characterization of this stabilization was reported to date. In this study, we photolyzed a mixture of 1,3-diiodo-but-2-ene and O2 to produce MVKO and characterized the syn-trans-MVKO, and tentatively syn-cis-MVKO, with transient infrared spectra recorded using a step-scan Fourier-transform spectrometer. The O‒O stretching band at 948 cm−1 of syn-trans-MVKO is much greater than the corresponding bands of syn-CH3CHOO and (CH3)2COO Criegee intermediates at 871 and 887 cm−1, respectively, confirming a stronger O‒O bond due to resonance stabilization. We observed also iodoalkenyl radical C2H3C(CH3)I upon photolysis of the precursor to confirm the fission of the terminal allylic C‒I bond rather than the central vinylic C‒I bond of the precursor upon photolysis. At high pressure, the adduct C2H3C(CH3)IOO was also observed. The reaction mechanism is characterized.