The catalytic oxidation of hydrogen chloride to chlorine (Deacon reaction) is an attractive means to recover Cl2 from HCl-containing waste streams in the chemical industry. Recently, RuO2 supported on TiO2 rutile has been industrially implemented for large-scale chlorine manufacture. However, detailed understanding of the reaction mechanism over claimed catalysts has not been acquired. Our tests in a fixed-bed reactor at ambient pressure concluded that RuO2 powder is highly active for Cl2 production. Characterization of the fresh and used RuO2 samples by ex situ XRD and XPS revealed no appreciable alteration of the bulk structure and limited chlorination of the catalyst. Ab initio thermodynamics predicted that the initial state of the RuO2(110) surface is partially over-oxidized, while the surface after reaction contains both oxygen and chlorine. DFT described the Deacon reaction with a Mars-van Krevelen type mechanism consisting of five steps: hydrogen abstraction from HCl, recombination of atomic chlorine, hydroxyl recombination, water desorption, and dissociative oxygen adsorption. An increased chlorine production was observed experimentally when increasing the feed O2-to-HCl ratio. This can be ascribed both to the lower recombination energy of chlorine atoms to gas-phase Cl2 at high coverages and the faster surface reoxidation due to higher partial oxygen pressures.