The removal of acetylene by selective hydrogenation is a critical step in the purification of ethylene streams for industrial polyethylene production. Hydrogenation of 0.35-1.0% acetylene in ethylene over Pd/Al\sb2O\sb3 catalysts is accompanied by the formation of significant amounts of surface oligomers/polymers. During the initial stages of CSTR operations at 40-120\sp∘C and P = 1 atm these accounted for 8 to 50% of the acetylene consumed. Ethylene selectivities calculated ignoring these polymers are incorrect. Approximately 30% of these products are volatile and soluble in hydrocarbon solvents. They consist of even carbon numbered chains from C\sb8 to at least C\sb30. Normal paraffins are the major species at each carbon number, although branched paraffins, linear and branched mono-and di-olefins, and alkylbenzenes are also produced. The remainder is nonvolatile and insoluble in any known solvent; its degradation products are consistent with those of polyacetylenes of carbon number greater than 24. A mechanism involving polymerization in regions of low surface hydrogen concentrations and terminated by hydrogenation is proposed. The accumulation of the liquids in catalyst pores imposes diffusion limitations on the acetylene reaction; both the rate of acetylene consumption and its selectivity to ethylene decrease. These effects are reversible upon removing the liquid polymers. The decrease in surface polymer selectivity observed during operation is compensated by an increase in gas phase oligomer selectivity; the total oligomer/polymer selectivity does not change appreciably. In conjunction with previous investigations showing nearly constant ethane production only from acetylene, this suggests that the product distribution from acetylene hydrogenation does not change significantly. All catalysts exhibited an induction period where the activity increased and the ethylene selectivity decreased, the latter due only to an increase in the rate of ethylene hydrogenation. The duration of this period increased sharply with decreasing catalyst metal loading, which is proposed to be inversely related to metal dispersion. The increasing duration of activation is therefore a form of structure sensitivity. Activation was only reversible upon moderate temperature hydrogen or oxygen/hydrogen treatments and represents the formation of some active surface species.