Ruthenium-based oxides with rutile structure were examined regarding their properties in electrocatalytic ethene oxidation in acid media. A possible promoting effect of chloride ions toward oxirane formation was explored. Online differential electrochemical mass spectrometry combined with electrochemical polarization techniques were used to monitor the potential dependence of organic products resulting from ethene oxidation as well as the reaction solution decomposition products. Quantum chemical modeling by means of density functional theory was employed to study key reaction steps. The ethene oxidation in acid media led to CO2, whereas oxirane was formed in the presence of 0.3 M Cl-. In the Cl- promoted oxidation on RuO2, oxirane and a small amount of CO2 were the only detected electro-oxidation products at potentials below the onset of Cl-2 and O-2 evolution, resulting from Cl and water oxidation. It is demonstrated here that the epoxidation is a surface-related electrocatalytic process that depends on the surface properties. Cl acts as the epoxidation promoter that switches off the combustion pathway toward CO2 and enables the epoxidation reaction channel by surface reactive sites blocking. The proposed epoxidation mechanism implies binuclear (recombination) mechanism for O-2 evolution reaction on considered surfaces.