The hydrogen oxidation reaction activity of sub-monolayer Pt-shell/Pd-core nanoparticles was investigated at a Pt surface concentration of 0 similar to 5.7%. The nanoparticles were prepared using a colloidal approach. The combination of electrochemical measurements and high resolution-X-ray photoelectron spectroscopy revealed a significant decrease in oxidized Pd atoms (23.4%) with a Pt surface concentration of 1.7% compared with that of Pd/C. X-ray absorption spectroscopy of the Pt LIII edge suggested preferred Pt deposition, which led to more oxidized Pt atoms during Pt shell growth. The exchange current density of the hydrogen oxidation reaction on the electrocatalyst with a Pt surface concentration of 4.9% was 3.5 times higher than that on Pd/C and was comparable with that on Pt surfaces. The changes in hydrogen oxidation reaction apparent enthalpy due to Pt shell growth suggested a rate determining step (RDS) change (Volmer reaction -> Tafel reaction) at a Pt surface concentration of 1.7%. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.058301jes] All rights reserved.