The hydrogenation of acetylene and acetylene-ethylene mixtures on a commercial palladium catalyst for hydrogenation of acetylene traces in ethylene-rich feedstock was studied. The obtained results suggest that during the reaction two types of catalytic sites are created by carbonaceous deposits on the palladium surface: A sites representing small spaces and E sites representing large spaces of palladium between the carbonaceous deposits. The A sites adsorb acetylene and hydrogen but, due to steric hindrance, do not adsorb ethylene. Consequently, these sites are involved in the conversion of acetylene, producing ethylene and small amounts of ethane and butadiene, and they are inactive in the hydrogenation of ethylene. Instead, the E sites adsorb all reactants and, therefore, they are responsible for the hydrogenation of ethylene and butadiene at low acetylene partial pressures.