X-Ray scattering has been used to study the structure of the Pt(1 1 1) interface with 0.1 M KOH and 0.05 M H2SO4 electrolytes. The absence of in-plane reconstruction peaks and the intensity profiles of the crystal truncation rods (CTR) indicate that the surface is not reconstructed at any potential. At potentials just above hydrogen evolution a monotonic expansion of the top Pt layer occurs in the region where hydrogen is adsorbed on the surface. The maximum expansion was approximately 0.03 Angstrom in KOH electrolyte and approximately half this magnitude in H2SO4 electrolyte. For both electrolytes this expansion is smaller than observed for the Pt(0 0 1) surface. The "anomalous voltammetric feature" observed in both electrolytes appears to be due to anion adsorption and/or an ordering transition in the inner ionic layer. At potentials approximately 1 V positive of hydrogen evolution, displacement of a fraction of the top Pt layer from bulk lattice positions occurs during anodic oxidation. This restructuring is reversible upon lowering the potential and reducing the oxide.