The electrical double-layer at a solid electrode does not m general behave as a pure capacitance but rather as an impedance displaying a frequency-independent phase angle different from 90 o Ways are indicated how to analyse the interracial impedance ,f such a complication arises in the presence of a faradalc process, both on the supposition that the double-layer behavlour is due to surface mhomogenelty and on the supposition that it is a double-layer property per se As examples, the equations derived are successfully apphed to a totally Irreversible and an ac quasi-reversible electrode process at a gold electrodeAccording to conventional double-layer theories, the impedance Z of an ideally polarized electrode consists of a capacitance (Cd) in series with the solution resistance (R e), and consequently the corresponding complex plane diagram (Z" vs. Z') should exhibit a straight vertical line intersecting the horizontal axis at Z' = R e, while the ordinates along this line equal Z" = (~0Cd) -a.This ideal behaviour is experimentally best observed at mercury/(aqueous) solution interfaces, for example at a dropping mercury electrode (DME), a hanging mercury drop electrode (HMDE), or a mercury pool electrode. Deviations from ldeality may occur in these cases, owing to rather trivial effects such as capdlary response or creeping [1], shielding by the glass capillary [2,3], and contamination of the mercury surface. The result of such effects is usually a curved complex plane plot, approaching the straight vertical line at higher frequencies. A less trivial deviation may be caused by diffusion-controlled adsorption of species present at a low concentration.Also at solid electrodes interfering effects, especially contamination and surface roughness, are hkely to be present, but apart from these a more marked behavlour 0022-0728/84/$03 00