•Using the method Pourbaix diagram we identified the oxygen covered RuO2(110) surface as the catalytically active phase under chlorine evolution reaction (CER) conditions. This active phase is compared with the active phase in the Deacon process, the heterogeneous gas phase counterpart of the CER. Constrained ab initio thermodynamics in the form of a Pourbaix diagram can greatly assist kinetic modeling of a particular electrochemical reaction such as the chlorine evolution reaction (CER) over RuO2(110). Pourbaix diagrams reveal stable surface structures, as a function of pH and the potential. The present DFT study indicates that the Pourbaix diagram in the CER potential region above 1.36 V and pH values around zero is dominated by a stable surface structure in which all coordinatively undercoordinated Ru sites (Rucus) are capped by on-top oxygen (Oot). This oxygen saturated RuO2(110) surface is considered to serve as the catalytically active phase in the CER, quite in contrast to the heterogeneously catalyzed HCl oxidation (Deacon process), for which the active RuO2(110) surface is mainly covered by on-top chlorine. The active sites in the CER are suggested to be RucusOot surface complexes, while in the Deacon process both undercoordinated surface Ru and oxygen sites must be available for the activation of HCl molecules.