The trimerization of acetylene to benzene on a Cu(100) surface is reported. The reaction occurs at both room temperature and at or below 200 K depending on the coverage. The reaction yield exhibits a strong coverage dependence, with the yield increasing as the initial acetylene coverage increases. The maximum yield for the room temperature reaction is 30%. The structural properties of the acetylene overlayer have been investigated by LEED, and the results correlated with the reaction yield observations. The results are discussed in light of the coverage dependence of the benzene yield for the same reaction on Cu(110) and Pd(lll). It is found that the structural arrangement of the acetylene overlayer on the three different surfaces explains the different coverage dependences of the reaction yield. A unique feature is observed in the thermal desorption data for this system; the mass 26 signal mimics the trimerization reaction rate at high coverages. This is most likely due to the exothermicity of the benzene reaction, leading to the desorption of vibrationally hot benzene molecules, or the transfer of energy to nearby acetylene molecules, resulting in simultaneous desorption.