Various Pd shell thicknesses (0.12-1.5 nm) were synthesized on preformed Au particles of 5 nm size by seeded growth technique (15-80 at% Pd) using sodium citrate and tannic acid. The sols were characterized by UV-vis spectroscopy, TEM and high-resolution TEM (HRTEM) measurements. HRTEM confirmed the pseudomorphic growth of the Pd shell on the Au core. The Au/Pd core/shell particles were fixed onto SiO2 (Aerosil 200) support by PDDA polycation. The catalytic activity in acetylene hydrogenation and selectivity of competition between acetylene and propene were tested after O-2 and H-2 pretreatment. The samples even in "as prepared" state hydrogenated acetylene. The thin Pd layer (15-30 at% Pd) on Au provided higher hydrogenation activity than the thick Pd shell. However, thermal treatment of the samples in H-2 stream causing Au/Pd intermixing shifted the activity maximum to higher Pd concentration (68-80 at% Pd). Comparison of the TOF (1/s) and selectivity values allowed us to conclude that the homogenized particle with 68-80 at% Pd shows better hydrogenation activity and selectivity than the thin Pd shell (15-30 at% Pd) on the Au core. (C) 2008 Elsevier B.V. All rights reserved.