Semi-continuous electroless deposition (ED) methods have been developed for preparation of variable and controlled coverages of Pt on Pd surfaces. The deposition of Pt occurred in an aqueous bath containing a reducible metal salt (PtCl62-), reducing agent (hydrazine) and stabilizer (ethylenediamine). To avoid electrostatic adsorption of PtCl62-, bath pH was controlled at pH 9.0, which was higher than the PZC of the carbon support, to create a negatively-charged carbon surface. Bath stability was maintained by addition of ethylenediamine and limiting the concentration of N2H4 in the bath to prevent thermal reduction of PtCl62- to form Pt-0. The concentration of N2H4 was controlled by pumping N2H4 solutions at various pumping rates into the ED bath. Thus, bimetallic Pt-Pd particles with Pt loadings of 6.0, 11.7, 17.2, and 22.7 wt% were selectively deposited on Pd surfaces of 30 wt% Pd/C. The structures of the catalysts were determined by STEM and EDS as variable thickness Pt shells with Pd cores. Pt loadings of 6.0, 11.7, 17.2, and 22.7 wt% corresponded to Pt shells of 0.9, 1.7, 2.7, and 3.4 monolayers (ML) on Pd. The catalysts were evaluated for their oxygen reduction reaction activity. The core-shell Pd-Pt/C catalysts were very active, especially the sample containing 0.9 ML Pt coverage on Pd with a mass activity of 329 A/g Pt compared to 183 A/g Pt for a conventional 50.5 wt% Pt/C sample. Similarly, electrochemical surface areas (ECSA) for all Pt shell samples (72-211 m(2)Pt/g Pt) were higher than for the conventional catalyst (58 m(2)Pt/g Pt). (C) 2016 Elsevier B.V. All rights reserved.