Supported Pd catalyst modified with Si, which is deposited selectively on Pd by silane decomposition and subsequently oxidized in oxygen, shows improved ethylene selectivity in acetylene hydrogenation. The origin of the selectivity improvement has been studied by observing the adsorption and desorption behavior of acetylene, ethylene, and hydrogen on the Pd surface. The adsorption strength of ethylene on I'd becomes weak and the amount of adsorbed hydrogen decreases when the Pd catalyst is modified with Si. The Si modification also reduces the amounts of surface hydrocarbons or carbonaceous species that are deposited on the catalyst either during the temperature-programmed desorption (TPD) of ethylene or by surface reactions between co-adsorbed acetylene and hydrogen. The hydrocarbon species deposited on the Si-modified catalyst have a shorter chain length than those produced on the Pd-only catalyst. All these results are consistent with the improvement in ethylene selectivity on the Si-modified Pd catalyst, which has been explained in this study based on the reaction mechanism of acetylene hydrogenation.