Although double-layer charging corrections to results of charge-transient and cyclic voltarmmetry current responses are required in experiments conducted at oxidized Pt electrodes, little information is available on the interfacial double-layer capacitance at such surfaces. Problems have arisen in both electro- reflectance and nanogravimetry studies related to the double-layer charging correction. In the present work, the interfacial capacitance of an oxidized polycrystalline Pt surface was determined by means of impedance spectroscopy at a series of descending controlled potentials in order to avoid time-dependent effects that otherwise arise due to oxide film growth. The experimental impedance data for an oxidized polycrystalline Pt electrode in 0.5 M H2SO4 or 0.5 M HClO4 between 0.9 and 1.4 V (RHE) were best described by a CPE with a phi value greater than 0.93, and increasing with increasing potential. The double-layer capacitance decreased from 80 to 40 muF cm(-2) between the potential limits of 0.9 and 1.4 V (RHE). Both phi and C were independent of added Cl- concentration within the concentration range 0-0.01 M. The variation in capacitance may be attributed to the diminishing contribution of anion adsorption pseudocapacitance as the potential is increased and/or residual pseudocapacitance due to a Pt2+/Pt4+ redox couple of the film. The double-layer capacitance at unoxidized Pt metal was also determined in 0.5 M H2SO4 (including 0, 0.001 M and 0.01 M Cl-) and in 0.5 M HClO4 (including 0, 0.001 M and 0.01 M Cl-) at 100 mV intervals in the double-layer charging region on Pt.