Infrared absorption spectra of methyl vinyl ketone (MVK, butenone) isolated in solid Ar were recorded. Irradiation of MVK at 308 nm partially converts antiperiplanar (ap-, previously called s-trans) MVK to synperiplanar (sp-, previously called s-cis) MVK; the difference spectrum clearly distinguishes absorption lines of these two conformers. Calculations using density functional theories (B3LYP with an aug-cc-pVTZ basis set) were performed to predict the geometry, energy, vibrational wavenumbers, and infrared intensities of ap- and sp-MVK. Experimental results on vibrational wavenumbers and relative intensities agree with theoretical calculations. To distinguish these conformers isolated in matrices, irradiation in situ is more effective than use of supersonic expansion during deposition.