Hydrogenation of acetylene-ethylene mixtures was studied on Pd/SiO2 and Pd-Ag/SiO2 under conditions that correspond to "front-end" hydrogenation of acetylene. The presence of excess H-2 under "front-end" hydrogenation conditions can lead to thermal runaway due to the exothermic reaction of ethylene and H-2. In previous work, we found that activation energy was sensitive to catalyst pretreatment: high temperature treatment in H-2 leading to lower apparent activation energy, while pretreatment under oxidizing conditions leading to higher activation energies for ethylene hydrogenation. Since ethylene hydrogenation is a. known structure-insensitive reaction, it was puzzling that the apparent activation energy should be so sensitive to catalyst pretreatment. As we show in this work, the presence of co-adsorbed CO (which is present during "front-end" hydrogenation), and changes in the surface structure of Pd-Ag surfaces together modify the apparent activation energy for ethylene hydrogenation. In situ infrared spectroscopy of adsorbed carbon monoxide reveals how the presence, of Ag modifies the proportion of bridged and linearly bound CO on Pd/SiO2. High temperature reduction further modifies the ratio of bridged to linear CO on Pd-Ag surfaces. Since CO in bridged form is bound more strongly than linearly bound CO, this shift in adsorption geometry modifies the apparent activation energy for ethylene hydrogenation. This work describes how restructuring of bimetallic Pd-Ag surfaces and the presence of a co-adsorbed CO, work together to alter the reaction behavior of an industrially significant selective hydrogenation reaction. (c) 2004 Elsevier B.V. All rights reserved.