The interfacial impedance for platinum-group metals in simple binary electrolytes, in their so-called double layer region, is generally assumed to be ideally capacitive. Using single crystalline platinum-group metals with various solutions we demonstrate that this is not necessarily the case when the anion of the solute is specifically adsorbed on the electrode. We attribute the frequency dependence of the interfacial capacitance to the relatively slow exchange of anions between the outer and inner Helmholtz planes. (C) 2007 Elsevier Ltd. All rights reserved.