Ab initio molecular orbital theory has been used to investigate the gas-phase reaction of Al+(S-1) with acetylene((1) Sigma(g)(+)). The energetics obtained with the refined GAUSSIAN-2 procedure reveal the existence of at least four experimentally distinguishable [Al,C-2,H-2](+) isomers on the singlet potential energy surface (PES) with the relative G2 energies in kcal/mol : Al+(HCCH) (1), 0.0; Al+(C(H)CH) (2), 25.9; H-Al+-CCH (3), 8.4; Al+(CCH2) (4), 28.8. Frequency analysis characterizes them unambiguously to be minimum structures. Additionally, the transition-state structures connecting the various minima have been determined. A comparison is made with regard to results of matrix studies on the neutral [Al,C-2,H-2] species and related ab initio calculations on the doublet PES. The differences of the acetylene((1) Sigma(g)(+)) --> vinylidene((1)A(1)) rearrangement in the presence of Al.(P-2), Al+(S-1) and in the metal-free case are discussed, and it is observed that analogous to the neutral aluminum, the cation catalyzes the isomerization process vinylidene((1)A(1)) --> acetylene((1) Sigma(g)(+)). Additionally, oxidative addition of acetylene to the Al+ center was found to be exothermic by only 6.6 kcal/mol, but it proceeds over a substantial barrier of at least 59.1 kcal/mol.