The reactivity of simple alkynes (acetylene, propyne, and 2-butyne) on various reduced TiO2 (001) surfaces was investigated using temperature-programmed desorption(TPD). The predominant reaction pathway in each case was cyclotrimerization to form aromatic compounds (benzene from acetylene, trimethylbenzene from propyne, and hexamethylbenzene from 2-butyne) with selectivities between 51% and 86%. Other less prevalent reactions included reductive dimerization to open chain dienes and hydrogenation to olefins. The selectivity for cyclization increased with reactant carbon number for the series above, while those of the olefin and diene products decreased, as did the fraction of the alkyne adlayer converted. By analysis of the cracking fractions of the trimethylbenzene produced during methylacetylene TPD, it was determined that the ratio of the 1,2,4- to 1,3,5-isomers was approximate to 3:1. This is the isomeric ratio expected if cyclotrimerization proceeds through a metallacyclopentadiene intermediate, with no preferential orientation of the methylacetylene molecules inserted. The yield of trimethylbenzene can be directly correlated with the population of Ti(2+) cations, quantified by XPS, on reduced TiO2 (001) surfaces. This surface site requirement is analogous to that for formation of metallacyclopentadiene complexes in solution; the latter reaction with mononuclear complexes requires transition metal cations capable of undergoing a two-electron oxidation. Direct precedents exist for formation of titanium(IV) cyclopentadienyl complexes from Ti(II) species in solution. Although the cyclotrimerization of alkynes has been reported on several single crystal metal surfaces, this is the first example of this reaction on a single-crystal metal oxide surface under ultrahigh-vacuum (UHV) conditions.