With density functional theory (DFT) calculations, we studied the oxidation of HCl with oxygen producing Cl2 and water on the TiO2(110)-supported RuO2(110). This so-called Sumitomo-a novel Deacon-process proceeds via a one-dimensional Langmuir?Hinshelwood mechanism, in which the recombination of two adjacent chlorine atoms on the surface of the catalyst constitutes the rate-determining step. Very important for industrial application is that substantial Ru resources can be saved in the production of the Sumitomo catalyst. According to our DFT calculations already 1 ML of RuO2(110) supported on TiO2(110) suffices to maintain practically the full activity of bulk-RuO2 in the HCl oxidation reaction. The calculated electron density differences of the TiO2(110)-supported 1 ML RuO2 system in comparison with bulk RuO2(110) are localized at the internal interface, leaving the electronic structure of the topmost undercoordinated Ru sites (active sites) unaffected by the support. This explains naturally the invariant activity of supported 1 ML RuO2(110) in comparison with bulk RuO2(110). The stoichiometric TiO2(110) is not active at all in the HCl oxidation reaction. However, if the undercoordinated Ti surface atoms are substituted by Ru then the resulting 1/2 ML RuO2-TiO2(110) catalyst is active with an activation barrier that is 58 kJ/mol higher than for bulk-RuO2(110).