The dissociation of nitric oxide on a ruthenium(0001) surface was studied by scanning tunneling microscopy. The distribution of nitrogen atoms after the dissociation allowed the identification of the "active sites" for this reaction, which are formed by the low-coordinated, too metal atoms of atomic steps. It is proposed that their activity is caused by local changes in the electronic structure. The structure of the steps deter mines whether they remain active or become deactivated by oxygen atoms. The results demonstrate the complex manner in which the structure of a catalytic surface determines the reactivity of the catalyst and confirm the active sites concept.