Small Ru- and Pt-containing mono- and bimetallic particles supported on silica were characterized using both theoretical thermodynamic simulations and experimental nuclear magnetic resonance (NMR) of chemisorbed hydrogen. The thermodynamic Monte Carlo simulations have provided detailed information on the morphologies, surface structures, and surface compositions of Ru–Cu and Pt–Cu bimetallic particles. For small Ru and Pt crystallites, a significant fraction of surface atoms populate defect-like edge and corner sites (e.g., 20% of total surface atoms for a particle with a dispersion of 0.31). These sites are preferentially occupied by copper. NMR of chemisorbed hydrogen is a surface-sensitive technique and has been used to measure the overall surface composition of the bimetallic particles and reveal subtle changes in the surface electronic environments due to addition of a second metal (copper) or a surface contaminant (chlorine or sulfur). The surface composition results determined from thermodynamic simulations are in agreement with those from proton NMR measurements for a series of 4% Ru–Cu/SiO2 and a series of 5% Pt–Cu/SiO2 bimetallic catalysts. Chlorine and sulfur are capable of inhibiting hydrogen chemisorption on Ru surfaces at a less-than-monolayer coverage. This observation and also the observed decrease in proton Knight shifts with increasing Cl coverage (less pronounced for S) on Ru surfaces were attributed to a short-ranged, ‘‘through-the-surface’’ electronic effect.