The effects of mass transfer on the electrocatalytic reduction of CO2 on a polycrystalline Cu rotating cylinder electrode were investigated. When the rotation rate was increased, the current efficiency toward CO2 reduction products decreased while that for the hydrogen evolution reaction increased. Furthermore, the product selectivity switched from CH4 to CO as the rotation rate was increased. This observation is generally inconsistent with the widely reported dependence of the electrocatalytic CO2 reduction on interfacial pH and CO2 concentration. As increasing the rotation rate improves mass transfer of species to and from the electrode surface, the interfacial pH becomes closer to the bulk pH while the interfacial concentration of CO2 at the electrode surface increases. However, increasing the rotation rate significantly decreased the CO2 reduction activity for constant current electrochemical CO2 reduction despite the increased availability of CO2 at the electrode surface. As the changes in interfacial pH and CO2 concentration with rotation rate cannot adequately explain the results, it is instead suggested that the enhanced mass transfer of dissolved CO away from the electrode surface at high rotation rates is the main reason behind the observed effects. We propose that this enhanced mass transfer of CO away from the electrode surface decreases the surface coverage of COads ( due to the equilibrium between COads and dissolved CO at the electrode-electrolyte interface) and limits the further reduction of COads to hydrocarbons. (C) 2017 Elsevier Ltd. All rights reserved.