The ion-molecule chemistry of C3H3+is systematically investigated with a series of alkenes, alkynes, and aromatic molecules under low-pressure, room temperature conditions. Based upon their reactivity differences, two C3H3+structures are distinguished and are assigned as the cyclic (the most stable) and linear isomers. Cyclic C3H3+readily reacts with unsaturated compounds having four or more carbon atoms. Linear C3H3+is found to be even more reactive and, in particular, forms condensation products with acetylene and benzene. The relevance of these results for the higher temperature and atmospheric pressure conditions of a flame environment is discussed. Since C3H3+has been found to be the dominant positive ion for rich and sooting hydrocarbon flames, its high reactivity provides a rapid first step in the ion models of soot formation.