Infrared laser spectroscopy and mass spectrometry were used to determine the structures of intermediates and products in the single-atom-catalyzed trimerization of acetylene to form benzene. Complexes of the form V+(C2H2)(n) were produced in the gas phase via laser ablation in a pulsed-nozzle source, size-selected with a mass spectrometer, and studied with infrared laser photodissociation spectroscopy. Density functional theory calculations were performed in support of the experiments. Single- and double-acetylene complexes form V+(C2H2)(n) metallacycle structures. Three-acetylene complexes exhibit a surprising dependence on the acetylene concentration, forming V+(C2H2)(3) and (C2H2)-V+(C4H4) tri- and dimetallacycle ion structures at low concentrations and eventually V+(benzene) at higher concentrations. These observations reveal intermediates along the reaction path of acetylene cyclotrimerization to benzene.