Almtraet-The effect of reconstruction of the Au( 100) surface on the double-layer capacity was measured usmg standard (IC methods combmed with UHV surface preparation and ex sttu structure anaiyas The (5 x 20) reconstruction present on the UHV-prepared surface was found to be stable m ddute HClO, over a mde range of electrode potential In dtlute H2S04, the (5 x 20) reconsmon was bfted at potentials near the pzc by specdic adsorption of HSO; even when the amon was present at very low concentrations, eg lo-' M In 0 01 M HClO,, the reconstruction was ldkd only at potentials 0 2 V postwe of the pzc, and was restored when the potential was made cu 0 1 V negafive of the pzc for the (1 x 1) surface, Y the (5 x 20) P (1 x 1) transformation was quasi-reverable m ddute HClO, Atormc scale roughness, eg steps, appeared to lower the stablhty of the (5 x 20) structure The capacity curve for the (5 x 20) reconstructed surface was dramatlcally different from that of the (1 x 1) surface, with a negauve shdI m the pzc of co 0 2 V followmg the transformation (5 x 20)+(1 x l), and a decrease m the capac~tancc of 20-25%, approximately equal to the change m atomic density of the surface The vacuum work fun&ons (@) of the emersed (5 x 20) and (1 x 1) surfaces were measured usmg photoenussron spectroscopy. and found to be 5 30 f 0 05, and 5 05 f 0 05 eV, respectively, which goes a A@ consistent urlth the relative values of the pzcs Our results were consistent m every respect urltb the previous findmgs of Kolb and co-workers on the effect of reconstruction on the double-layer capactty of Au(100) surfaces