The formation of ethylene in CO2 electroreduction over rough copper electrodes is often explained by the presence of specific surface crystal steps, edges and defects. We demonstrate that an identical electrode covered with copper nanoparticles can yield either predominantly ethylene or methane, depending on the electrolyte concentration and applied CO2 pressure. Calculations of the pH near the electrode surface suggest that ethylene formation is favored by a relatively high (local) pH. Furthermore, the conditions leading to the formation of significant amounts of methane result in rapid deterioration of hydrocarbon production rates, whereas electrode performance in conditions favoring ethylene production can be sustained for hours. This study substantially alters the mechanistic interpretation of formation of ethylene over rough copper surfaces and implies that applied process conditions inducing pH variations near the electrode surface need to be taken into consideration.