Capacitive deionization (CDI) and membrane capacitive deionization (MCDI) are widely considered as promising, highly energy efficient processes for water desalination, of which commonly used performance indicators are the average salt adsorption rate, the salt removal efficiency, and the charge efficiency. Quantification of the sustainability performance of CDI and MCDI is still scarce, and in this paper, we use exergy analysis to evaluate the resource use efficiency of membrane capacitive deionization (MCDI). The electric as well as chemical exergies of the salt solution, and the stored ions, are used to calculate the exergy efficiency (eta(ex)) and cumulative exergy losses (CEL) ranging between 2 to 13% and 0.5 to 8 J/mol water, respectively. From an exergetic point of view, passive adsorption in combination with active desorption (-0.9 V) is favorable, yielding the highest eta(ex) and lowest CEL values. The combination of active salt adsorption using an electric field, with either passive or active desorption gives higher productivities, but at the cost of a disproportionate amount of exergy (and energy) input.