Porous electrodes are important in many physical−chemical processes including capacitive deionization (CDI), a desalination technology where ions are adsorbed from solution into the electrostatic double layers formed at the electrode/solution interface inside of two juxtaposed porous electrodes. A key property of the porous electrode is the charge efficiency of the double layer, Λ, defined as the ratio of equilibrium salt adsorption over electrode charge. We present experimental data for Λ as a function of voltage and salt concentration and use this data set to characterize the double-layer structure inside of the electrode and determine the effective area for ion adsorption. Accurate experimental assessment of these two crucial properties of the electrode/solution interface enables more structured optimization of electrode materials for desalination purposes. In addition, detailed knowledge of the double-layer structure and effective area gives way to the development of more accurate dynamic process models describing CDI.