Ag(3 wt%)-loaded γ-Al₂O₃ (Ag/Al₂O₃) catalysts were prepared using four types of commercially available alumina powders (CTB, PUR, VGL, and CFF). Based on the support, the activity of these catalysts for the H₂-assisted selective catalytic reduction (SCR) of NO by NH₃ or C₃H₆ decreased in the order CTB > PUR > VGL > CFF. After sintering treatment (H₂ reduction at 800 °C), the particle size of the Ag metal nanoparticles (NPs) changed and was found to be correlated with the catalytic activity (CTB < PUR < VGL < CFF). After re-oxidation of H₂-reduced Ag/Al₂O₃ at 500 °C, the in situ infrared (IR) spectra showed negative bands at 3762 cm⁻¹ due to the HO-μ₁-Al_VI site, where the band intensity increased in the order CTB > PUR > VGL > CFF. IR study of pyridine adsorbed on Ag-free γ-Al₂O₃ showed that the number of strong Lewis acid sites (unsaturated Al_IV³⁺) increased in the same order, CTB > PUR > VGL > CFF, and the number of strong Lewis acid sites decreased when Ag was loaded on the supports. In situ X-ray absorption near-edge structure (XANES) and UV-vis studies of Ag/Al₂O₃ sintered under NO + O₂ at 400 °C showed oxidative redispersion of the Ag metal NPs to regenerate atomic Ag(I) sites. The amount of redispersed Ag metal and the initial rates of redispersion estimated from the in situ UV-vis results changed in the following order: CTB > PUR > VGL > CFF. These results suggest that the HO-μ₁-Al_VI site adjacent to the unsaturated Al_IV³⁺ site on γ-Al₂O₃ is the anchoring site of the atomic Ag species, and the sintering resistance of Ag/Al₂O₃ increases with the number of HO-μ₁-Al_VI sites. During H₂-assisted SCR, where both H₂ and NO + O₂ were co-fed to the catalysts, the number of highly dispersed Ag species (active sites) increased with the number of HO-μ₁-Al_VI sites; hence, NO conversion increased with the number of HO-μ₁-Al_VI sites on the support. The present results provide molecular-level insights into the design of sintering-resistant Ag/Al₂O₃ catalysts for SCR.