The chemical reactivity of SiOx species on Cu(100) and oxygen-passivated Cu(100) surfaces has been studied by means of synchrotron photoemission (PES), X-ray photoemission (XPS), X-ray absorption (XAS) spectroscopies and ab initio density functional calculations. When silicon monoxide is deposited on a clean Cu(100) surface, below the equivalent coverage of ∼2 Å, partial dismutation of Si2+ species into Si0 plus Si4+ species takes place. The presence of Si0 as patches of Si–Cu(100) surface alloy and SiO monomers and polymeric forms of Si2+ species has been explained theoretically. The observation of Si4+ species is explained by a two-step process: first the reduction of Si2+ to Si0 plus O adsorbed on the Cu(100) surface; second the adsorbed O reacts with the incorporated SiO to oxidised to Si4+. The mechanism would be similar to that proposed for the reaction CO→CO2 on O-Cu(100) surface. On the other hand, when very low coverages (equivalent thickness<0.5 Å) of SiO are deposited on an oxygen-passivated Cu(100) surface, only Si2+ species are stabilised, probably in an (SiO)n polymeric form. The theoretical calculations have confirmed very weak interaction between the SiO structures and the passivated surface. The Si-K absorption edge of these Si2+ species is characterised by only two peaks at 1840 and 1849 eV. They have been assigned to the π and σ orbitals of the SiO bond, by similarity with the CO molecule. Preferential orientation of the SiO units parallel to the surface is found by XAS and confirmed by the theoretical calculations.