Graphical abstract The pH value showed significant effect on the silver content and the species state, consequently affecting the catalytic activity of Ag/SBA-15 catalysts for CO oxidation. The pH adjustment of synthesis gel to 5 induced the formation of the Ag species with higher amount inside the channels and the higher activity for low-temperature CO oxidation, giving 100% CO conversion at 70 °C. However, the further increasing of the pH value resulted in more Ag 2O clusters formed inside the channels, leading to a decrease of catalytic activity for CO oxidation. Highlights ► The pH adjusting to 5 and 7.5 increased the silver loading inside the SBA-15. ► The silver catalyst prepared under pH = 5 condition gave 100% CO conversion at 70 °C. ► The pH higher than 5 induced the formation of more Ag 2O, giving the low activity.