Ag/alumina catalysts with different silver contents for octane-SCR were prepared by impregnation and incipient wetness methods. Activity tests revealed that the decisive factor for high activity is not only a high dispersion of silver, but also the ability of the system to redisperse clustered silver. Determination of dispersion by TEM/HAADF and O2-chemisorption experiments resulted in values close to each other even if the results were not directly comparable. This is suggested to be due to not complete silver reduction below 700 degrees C and the samples being very heterogeneous in terms of particle size, e.g., having a bimodal size distribution. Small charged Agsigman+ clusters containing 2-8 silver atoms highly prevailed in the samples containing <2 wt% Ag and exhibiting high octane-SCR activity. In highly loaded Ag/alumina samples or those reduced and reoxidized at high temperature (>400 degrees C), large metallic particles are stabilized, resulting in poor conversion of NOx to N2.