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Abstract Herein, aqueous nitrate (NO₃⁻) reduction is used to explore composition‐selectivity relationships of randomly alloyed ruthenium‐palladium nanoparticle catalysts to provide insights into the factors affecting selectivity during this and other industrially relevant catalytic reactions. NO₃⁻reduction proceeds through nitrite (NO₂⁻) and then nitric oxide (NO), before diverging to form either dinitrogen (N₂) or ammonium (NH₄⁺) as final products, with N₂preferred in potable water treatment but NH₄⁺preferred for nitrogen recovery. It is shown that the NO₃⁻and NO starting feedstocks favor NH₄⁺formation using Ru‐rich catalysts, while Pd‐rich catalysts favor N₂formation. Conversely, a NO₂⁻starting feedstock favors NH₄⁺at ≈50 atomic‐% Ru and selectivity decreases with higher Ru content. Mechanistic differences have been probed using density functional theory (DFT). Results show that, for NO₃⁻and NO feedstocks, the thermodynamics of the competing pathways for N–H and N–N formation lead to preferential NH₄⁺ or N₂production, respectively, while Ru‐rich surfaces are susceptible to poisoning by NO₂⁻feedstock, which displaces H atoms. This leads to a decrease in overall reduction activity and an increase in selectivity toward N₂production. Together, these results demonstrate the importance of tailoring both the reaction pathway thermodynamics and initial reactant binding energies to control overall reaction selectivity.