Abstract Capacitive deionization (CDI) is an electrochemically controlled desalination technology based on applying a non-faradaic process in the electrical double layer region. Because CDI performance is affected by the type of operational mode, this study examined salt adsorption capacity (deionization capacity) and energy consumption of two CDI operational modes, constant voltage (CV) and constant current (CC). The use of higher cell voltage in CV mode resulted in faster salt adsorption under a given charging time due to stronger electrostatic attraction than CC mode. However, despite of the faster salt adsorption of CV mode, CC mode showed much lower energy consumption than CV mode by 26%–30% as an identical electrical charge was consumed or an identical amount of ion removal was considered. This lower energy consumption is due to the overall lower cell voltage used in CC mode than in CV mode. This study suggests that better salt adsorption capacity and energy consumption in CDI operation may be achieved by applying appropriate CV and CC modes.