The oxidations of carbon monoxide and formic acid at ultrathin Pd layers grown on Au nanoparticles were studied as a function of Pd thickness. Pd shells with thickness between 1 and 10 nm were grown on 19 nm Au nanoparticles by chemical reduction of H2PdCl4 with ascorbic acid. High-resolution transmission electron microscopy and X-ray diffraction confirm the core–shell configuration of the nanostructures. While the synthesis of pure Pd nanostructures led to a rather amorphous material, Pd nanoshells exhibited a polycrystalline structure confirming that Au nanostructures act as templates for Pd growth. Three-dimensional assemblies of nanoparticles were generated by alternate electrostatic layer-by-layer adsorption steps, involving poly-l-lysine and colloidal dispersions. Electrochemical studies in H2SO4 containing electrolyte solution demonstrate that CO coverage and anodic stripping potential are affected by the thickness of Pd nanoshells. In addition, the faradaic current density associated with HCOOH oxidation significantly increases with increasing Pd thickness. The thickness-dependent reactivity of Pd nanoshells is discussed in terms of lattice strain relaxation.