The effects of variation of the rhodium dispersion on the catalytic reduction of NO by CO over Rh/Al2O3 catalysts have been investigated. NO dissociation is promoted by an increase of metal particle size which results in an enhanced catalytic activity on a turnover basis. However, the promotional effect of the particle size on the activity strongly depends on the reaction temperature, since under reaction conditions the oxidative disruption and reductive agglomeration processes modify the rhodium particle size. These processes appear to be responsible for changes of the apparent activation energy observed around 500 K.