Solid phosphoric acid catalyzed oligomerization of 1-butene rich material yielded highly branched products that were not expected from a standard carbocation mechanism. This peculiar oligomerization behavior of 1-butene has been noted previously in the literature and the present study deals with its explanation. Evidence for a low-temperature skeletal isomerization pathway of 1-butene to isobutene is presented. Although double bond isomerization caused equilibration of the n-butenes, there was a significant period during which 1-butene determined the product properties and oligomerization rate. During this period double bond isomerization was stereospecific, favoring cis-2-butene, and the ratio of trimethylpentenes to total C-8 olefins did not change. Macroscopic evidence indicated that a monomolecular, or pseudo-monomolecular mechanism was active for isomerization and an explanation that did not involve the formation of a primary carbocation could be presented.