Selectivity is a must for new high performance catalysts which are required to fit strict environmental regulations. RuO(2) has been proposed as a mild oxidant capable of performing selective oxidations of alcohols to aldehydes and ketones. However, it fails in another appealing partial oxidation, that of ethylene to its epoxide, where complete oxidation occurs. In contrast, both reactions are selective on Ag, a catalyst that shows similar binding energies for the active oxidation species, oxygen. By means of density functional theory applied to slabs we have been able to unravel the origin of the, in principle erratic, selectivity of RuO(2) in partial oxidations. The behavior can be understood as being due to a combination of the basicity of O atoms, the relative binding energies of reactants and products to the surface (thermodynamic factor), and the structure sensitivity of some steps in the oxidation process due to difficult O mobility on RuO(2).