The deactivation behavior of Si-modified Pd catalysts in acetylene hydrogenation was studied using thermogravimetric analysis (TGA), infrared (IR) spectroscopy, and reaction tests. TGA and IR analyses of green oil produced on the catalyst indicate that it is produced in smaller amounts and its average chain length is shorter on a Si-modified catalyst than on an unmodified one. The above findings are due to deposition of Si species on the Pd surface; such deposits effectively block multiply-coordinated adsorption sites on the catalyst and suppress the formation of green oil on the catalyst surface, specifically on or in the vicinity of Pd. The Si species also retard the sintering of Pd crystallites during the regeneration step and allow for the slow deactivation of the catalyst during acetylene hydrogenation, after regeneration. The improvement in the deactivation behavior of the Si-modified catalyst is believed to arise from the geometric modification of the Pd surface with small clusters of the Si species. (C) 2003 Elsevier B.V. All rights reserved.