We report the electrocatalysis of the chlorine evolution reaction (CER) on well-defined RuO2(110) and IrO2(110) surfaces. RuO2 and IrO2 are known for their capabilities to catalyze the CER. Until now, the CER measurements have only been reported on well-defined RuO2 surfaces and only at high Cl- concentrations. We present the CER measurement and the role of Cl- at lower concentration on single-orientation RuO2(110) and IrO2(110) films. We find that RuO2(110) is two orders of magnitude more active than IrO2(110). Moreover, we observe the correlation between the CER activity and the Oad formation potential on RuO2 and IrO2, supporting the prior suggestion that the Oad is the active site for the CER. We further use the reaction order analysis to support the Volmer-Heyrovsky mechanism of the CER, which was previously suggested from the Tafel slope analysis. Our finding highlights the importance of the surface Oad species on oxides for the CER electrocatalysis and suggests the electrochemical formation of Clad on Oad (for example, Cl- + Oad ↠OClad + e-) as the crucial step in the CER electrocatalysis. © 2019 Author(s).