Abstract The interfacial impedance for the platinum-group metals, in simple binary electrolytes, in their so-called double layer region, is generally assumed to be ideally capacitive. In this communication, mainly addressing the Ir(1 0 0) / 0.1 M HCl system, we demonstrate that this is not necessarily the case when the anion of the solute is specifically adsorbed on the electrode. The frequency dependence is attributed to the relatively slow exchange of anions at the outer Helmholtz plane with the adsorbed ones (i.e. those at the inner Helmholtz plane).