D-band position is often used to describe the reactivity of alloy catalysts. However, the spatial separation (dispersion) and geometric arrangement of catalytically active surface atoms can also control the system’s overall reactivity. My talk will discuss how the dispersion of catalytically active Rh atoms within 2-3 nm, ligand-protected Au/Rh alloy nanoparticles directly impacts electrocatalytic H₂ oxidation reaction (HOR) activity. The HOR is an interesting model reaction for the Au/Rh system because Rh atoms catalyze H₂ oxidation, whereas Au atoms do not. This provides the opportunity to study the composition-dependent evolution of HOR activity as a function of alloy composition (0-100% Rh). We find that HOR activity follows a volcano-type trend. Pure Au particles do not oxidize H₂, and peak HOR activity was found for particles containing 21% and 34% Rh. Beyond this optimum Rh content the HOR activity decreased towards that of 100% Rh particles and a commercially available Rh/C catalyst. In fact, HOR activity decreased as a function of (%Rh)^-1 beyond peak activity. We rationalize this trend through the dispersion of Rh-containing active sites on the surface of the Au/Rh alloy NPs. Analysis of literature results has identified similar inverse relationships between catalytic activity and composition. First-principles density functional theory (DFT) studies were also used to model H-binding at realistic Au/Rh alloy nanoparticles. H-binding energy is a common metric for predicting HOR activity. We found that Rh incorporation increased the d-band position of Rh containing particles compared with Au. However, did could not identify any relationship between H-biding and d-band energy. These results are in contrast to what would be expected from a purely electronic d-band model, and they provide strong evidence that intra-particle active site dispersion can tune the catalytic activity of alloy nanocatalysts. We hypothesize this phenomenon should also extend to other alloy systems, and active site dispersion provides another tool for controlling the nano-scale chemistry of electrocatalysts.