Dendrimer-encapsulated Au nanoparticles comprised of an average of 147 atoms were synthesized and immobilized on a glassy carbon electrode. A one-atom-thick shell of Cu was added to the Au core by electrochemical underpotential deposition, and then this shell was replaced with Pt by galvanic exchange. The results indicate that this synthetic approach leads to well-defined core/shell nanoparticles <2 nm in diameter. The rates of oxygen reduction at the Au@Pt electrocatalysts were compared to Pt-only and Au-only, 147-atom dendrimer-encapsulated nanoparticles.