The hydrogenation of acetylene-ethylene mixtures over a Pd/alpha-Al2O3 catalyst was studied. Kinetic investigations were carried out in a flow system equipped with a gradientless microreactor over a wide range of acetylene and ethylene partial pressures. Three types of active sites were thus identified : A(1) and A(2) sites which are highly selective in the hydrogenation of acetylene to ethylene via a mechanism involving the competitive (A(1)) and noncompetitive (A(2)) adsorption of acetylene and hydrogen, and E sites which are active only for the hydrogenation of ethylene. E sites are able to adsorb acetylene, an effect which strongly inhibits ethane formation. The above three types of active sites are very likely created on the palladium surface by carbonaceous deposits formed during the stabilization process. The identified sites determine the selectivity of the catalyst and manifest themselves in kinetic investigations depending on the process conditions.