A catholyte-free membrane electrode assembly (MEA) has been proposed for practical application in the electrochemical CO2 reduction reaction (eCO2RR), and water management becomes critical in its catalyst-membrane interface. We investigate roles of the water supply within the MEA for ethylene production by utilizing deuterium-labeled water. The protons of ethylene originated mainly from the anolyte not the humidified water through the cathode, indicating that dominant water flux from the anolyte acts as a major proton supplier for the eCO2RR. Meanwhile, humidification of CO2 is still important in the Faradaic efficiency and current density because it affects the water activity at the catalyst junction, supported by multiphysics simulations. At low cell potentials, the eCO2RR dominates and is kinetically controlled, and the mass transport of CO2 and water limits its performance as the potential increases. This understanding of the water kinetics and transportation provides valuable insights into the design of active MEAs.