Previous investigations on membrane capacitive deionization (MCDI) cells focused on electrode and membrane materials; while little attention has been paid to the separator compartment. This study demonstrated that the MCDI's desalination rate can be enhanced by inserting activate carbon fibers (ACF) in the cell's separator compartment. For comparison, stainless steel fibers (SSF) and filter paper (FP) were also examined as the separator. The highest desalination rate [513.4 mg/(L.h)] was obtained when ACF was inserted, followed by SSF [374.1 mg/(L.h)] and FP [297.9 mg/(L.h)]. The pseudo-first-order kinetic model provided a good approximation (R-2 > 0.99) to the measured adsorption data, and the model fitting results revealed that the MCDI with the ACF separator delivered not only the highest desalination rate but also the largest adsorption capacity. Electrochemical impedance spectrometry (EIS) analysis revealed that the ACF separator considerably reduced the MCDI's internal resistance and accordingly increased its desalination rate. Our findings highlight the need for revisiting and modifying the current designs of the MCDI's separator compartment. (C) 2015 Elsevier B.V. All rights reserved.