The valence-shell electronic structure of isobutene has been investigated by performing molecular (e, 2e) reactions in the symmetric noncoplanar geometry. Experimental momentum distributions (MDs) of selected ionic states have been determined and compared with ab initio calculations using self-consistent-field wave functions of 4-31G, 6-31G, and 6-31++G** basis sets. These MD measurements can be used to provide detailed information not only for the evaluation of the quality of ab initio wave functions but also for the assignments of two of the more prominent ionic states in the inner-valence region. Furthermore, together with our recent (e, 2e) work, the electronic structure and chemical bonding morphology of the isomeric series iso-, cis- and trans-butene can be investigated from the unique momentum-space perspective. In particular although the calculated orbital energies for the corresponding orbitals appear to be similar among the butene isomers, the measured and calculated MDs are found to be qualitatively different, particularly for valence-shell orbitals involving in-plane bonding overlaps in these isomers. The sensitivity of MDs to the different isomeric arrangements of the methyl groups in butene will be discussed in terms of momentum-space chemical concepts.