The design of palladium-based catalysts with improved selectivity, while maintaining high activity in hydrogenation reactions and avoiding the use of lead or ligands is still a major concern in heterogeneous catalysis. Here, a novel protocol has been proposed to prepare colloidal PdCu(y )nanoparticles with tailored metal composition, enabling a remarkable improvement in the semihydrogenation of alkynes when compared to pure Pd or a physical mixture of Cu and Pd catalysts. The PdCuy bimetallic catalyst with 1:4 Pd/Cu atom ratio showed the best performance. The synergistic effect between the closely contacting Pd and Cu species resulted in full conversion of alkyne and high alkene selectivity for a broad range of multifunctionalized substrates under very mild conditions (30 degrees C and 1 bar of H-2).