The reaction products formed from 1,1-diiodoethane adsorbed on clean Ag(111) have been investigated by using reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) and compared to cis- and trans-3,-butene adsorbed on the same Ag(111) surface. Ethylidene species synthesized from 1,1-diiodoethane adsorbed on Ag(111) dimerize to form 2-butenes. The TPD profiles of the reactively formed 2-butenes resemble those of cis- and trans-2-butenes directly adsorbed on Ag(111) exhibiting a sharp peak at similar to 140 K and a broad feature at similar to 184 K. The surface infrared spectra of cis- and trans-2-butene are distinguishable where monolayer cis-2-butene/Ag(111) exhibits peaks at 1445, 1434, and 1030 cm(-1) whereas the trans isomer has features at 1429, 973, and 959 cm(-1). It is found that the surface infrared spectrum collected after 1,1-diiodoethane adsorption on Ag(111) is very close to that of trans-2-butene on Ag(111). This demonstrates that the stereoselectivity of ethylidene dimerization on Ag(111) surfaces is controlled by the initial approach of the carbenes rather than the desorption of the 2-butene product.