In this paper the relation between cation mobility and catalytic activity in lean SCR NOx by CH4 on (Ag,Co)- and (Co,Ag)-FER have been studied by combining XRD Rietveld refinement in fresh and used catalysts with SCR catalytic testing performed in dry and wet conditions. UV-Vis DRS measurements were also performed. Maximum NOx dry conversion (40% at 500°C) in Co-FER is related to the temperature induced migration of Co2+ ions into catalytically active Co2a sites. The irreversible loss of catalytic activity in the presence of water is partly suppressed by first exchanging Ag cations, resulting in different cation siting distribution and the reduced mobility of Co cations which are responsible for SCR activity and selectivity. In particular, we propose that with (Co,Ag)-FER(Co 4.6, Ag 0.2 wt%) the absence of Ag+ in the Ag2 site could hinder Co migration to the Co2a site, and that of Ag0, which promotes the side oxidation of NO to NO2, together with an increase in the Co4 population being lost from Co2, could be responsible for preserving catalytic activity and selectivity.