There are 72 isomers of the octene molecules and only a small number of them have high value as a blending octane component in gasoline. The amorphous silica alumina catalyst used for selectively dimerizing octenes from linear butenes can itself isomerize the target species. It is demonstrated that this process of equilibration leads to an overall reduction of the blending octane number. This is dependent on the different reactivities of octene isomers over the catalyst under dimerization conditions. The boundary between double bond and skeletal isomerization is explored. The linear octenes have a much higher threshold for isomerization than the branched (high octane value) species. In addition, those higher order oligomers (almost exclusively trimers and tetramers) with a large number of allylic hydrogens are considerably more reactive, but this is partially offset at higher temperatures by cracking back down to dimers. A mixture of selective dimerization and associated oligomer cracking emerges as the most promising route for upgrading a mixed butene monomer feed.