As recently demonstrated, the morphology of a surface can be modified on the mesoscopic scale by ion sputtering. Here we show by microscopy and spectroscopy that the chemical properties of the surface are strongly affected by nanostructuring and that surface reactivity can be tuned by changing surface morphology. For the otherwise inert Ag(001) surface significant O-2 dissociation takes place on the nanostructured surface, thus allowing us to control the relative coverage of admolecules and adatoms. The dissociation probability is determined by the experimentally tunable density of kinks. (C) 2000 American Institute of Physics. [S0021-9606(00)71415-2].