Rh@Ptcore-shell, RhPt (1:1) alloy, and Rh + Pt monometallic nanoparticles (NPs) were prepared using standard polyol reduction chemistry in ethylene glycol (EG) with standard inorganic salts and polyvinylpyrrolidine (PVP55000) stabilizers. PVP-free colloids were also prepared but less stable than the PVP-protected NPs. Rh@Pt core-shell particles were prepared from 2.7, 3.3, and 3.9 nm Rh cores with varying shell thicknesses (similar to 1 and similar to 2 ML). The particles were characterized by a combination of TEM, single-particle EDS, EDS line scans, XRD analysis, Debye Function simulations, FT-IR, and micro-Raman CO-probe experiments. The three different architectures were evaluated for preferential oxidation of CO in hydrogen (PROX) using 1.0 wt % Pt loadings in Al2O3 supports. For hydrogen feeds with 0.2% CO and 0.5% O-2 the Rh @ Pt NP catalyst has the best activity with complete CO oxidation at 70 degrees C and very high PROX selectivity at 40 degrees C with 50% CO conversion.