The influence of open steps on the surface properties is shown by investigating the interaction of molecular ethene with Cu(410). We find a surprisingly low-temperature, site-selective chemistry at the strongly under-coordinated step sites. Ethene bonds either in a π-bonded or in a di-σ-bonded state or undergoes complete dehydrogenation. All pathways involve the low-coordination sites at the step, since the first species is partially stabilized with respect to low-Miller-index surfaces, while the other two are observed only on Cu(410). When annealing the surface, dehydrogenation and transformation into the di-σ-bonded moiety proceed, both processes being favored by faster heating rates. The so-generated carbon (presumably C2 admolecules) decorates the step edges, thereby blocking the active sites for subsequent dissociation and permitting only π-bonding of ethene. The dipole loss of carbon disappears in high-resolution electron energy loss spectroscopy when annealing to room temperature, indicating that carbon moves to more coplanar or even to subsurface sites where it still influences the surface chemistry. The surface reactivity is recovered when heating the crystal to 900 K since C dissolves then deep enough into the bulk.