Abstract Using density functional theory we study the effect of pre-adsorbed atoms on the dissociation of N 2 and the adsorption of N, N 2, and CO on Ru(0001). We have done calculations for pre-adsorbed Na, Cs, and S, and find that alkali atoms adsorbed close to a dissociating N 2 molecule will lower the barrier for dissociation, whereas S will increase it. The interaction with alkali atoms is mainly of an electrostatic nature. The poisoning by S is due to two kinds of repulsive interactions: a Pauli repulsion and a reduced covalent bond strength between the adsorbate and the surface d-electrons. In order to investigate these different interactions in more detail, we look at three different species (N atoms, and terminally bonded N 2 and CO) and use them as probes to study their interaction with two modifier atoms (Na and S). The two modifier atoms have very different properties, which allows us to decouple the different types of interactions. Adsorbed Na induces large electrostatic fields, which S does not, and S interacts strongly with Ru(4 d) states and broadens and shifts the d band, which Na does not do.