A monometallic Pt/SiO2 and two bimetallic Pt-Cu/SiO2 with Pt/Cu atomic ratio of 1 (Pt1Cu1) and 0.33 (Pt1Cu3) have been investigated by a combination of reaction kinetics and FTIR spectroscopic studies in order to understand the factors that control the selectivity toward ethylene and ethane in the CH2Cl-CH2Cl + H-2 reaction. Carbon monoxide adsorption was used to probe the electronic state (ligand effect) of Pt and Cu, and the selective poisoning of Pt sites with CO under catalytic reaction conditions was employed to elucidate the impact of the ensemble effect and the role of Pt and Cu sites in the conversion of 1,2-dichloroethane. It was shown that both Pt and Cu in the bimetallic Pt-Cu catalysts are modified electronically, but this modification does not impact the catalysts' selectivity patterns. However, the addition of CO into the CH2Cl-CH2Cl + H-2 reaction mixture at 473 K to block Pt sites resulted in a significant improvement in the ethylene selectivity of the bimetallic catalysts at the expense of ethane. The observations are consistent with the idea that with the Pt-Cu catalysts, ethylene forms on Cu sites and the role of Pt is to produce a limited supply of dissociated H atoms that spill over to Cu and react thereon with adsorbed Cl atoms.