The unimolecular rearrangement reactions of the isomeric enolate ions of 2-butanone have been investigated using Fourier transform ion cyclotron resonance mass spectrometry and infrared multiple photon activation techniques. The individual isomers of 2-butanone enolate ions were generated from the corresponding trimethylsilyl enol ethers and photodissociated independently. Infrared multiple photon activation of the 2-butanone enolate ions induces a 1,3-hydrogen rearrangement which interconverts the individual isomers. Because infrared multiple photon activation only involves vibrational excitation, the 1,3-hydrogen rearrangement must be a thermal reaction and occur on the ground electronic state potential energy surface. The observation of a 1,3-hydrogen rearrangement is unexpected and appears to violate the Woodward-Hoffmann symmetry rules. Orbital correlation diagrams show, however, that thermal suprafacial 1,3-hydrogen rearrangements are allowed for enolic systems. Nevertheless, a more probable reaction mechanism involves the rotation of the methylene groups so that the 1,3-hydrogen rearrangement corresponds to a simple proton transfer between two unsaturated carbons.