Ultrafast electron diffraction is reported for the elimination reaction of acetylacetone. For this multipath reaction, upon 266 nm excitation, it is revealed that the major reaction channel is fragmentation from the beta carbon, producing OH and 3-penten-2-on-4-yl radicals, with a time constant of 247 +/- 34 ps. We determine the structures involved and the changes in bond lengths. These structure-dynamics correlations in different states provide a mechanistic description of the complex reaction landscape.