The oligomerization of a mixed butene stream in liquid phase over amorphous aluminosilicates was investigated. The reaction products were analyzed with gas chromatography with special attention to the octene isomers. By varying temperature, content of iso-butene, and space velocity, the origin of skeletal isomers of octene was identified. Some octene isomers have been considered in the literature as a consequence of methyl group migration or as a cracking product. The catalytic results, however, indicate a direct formation of these C8 side products during dimerization. The correlation of catalytic measurements and thermodynamic data, calculated by quantum-chemical methods on the explicitly correlated coupled cluster single, double, and perturbative triple excitations level (CCSD(F12*)(T)), prove a competitive reaction between the formation of 4,4-dimethylhex-2-ene (4,4-DMH) and the main product trans-3,4-dimethylhex-2-ene (3,4-DMH). 3-Ethyl-2-methylpentene is also formed in competition with 3,4-dimethylhexene. The skeletal isomers 2,3-dimethylhexene and 2,4-dimethylhexene show a dependence on the proportion of 1-butene at the acid site, whereas a rapid double bond isomerization strongly influences the proportion of 1-butene. (C) 2018 Elsevier Inc. All rights reserved.