We present a novel model system for nanoparticle electrocatalysts. A surface consisting of alternating (100) and (111) facets, several nanometers across and nearly 1 mu m long, were self-assembled by annealing Pt single crystal surfaces initially cut at the midpoint between [111] and [100] directions, i.e., Pt(1+root 3 1 1). The formation of these self-assembled arrays of nanofacets was monitored by in-situ surface X-ray scattering. These surfaces were further characterized with scanning probe microscopy and cyclic voltammetry. We found that the Pt(1+root 3 1 1) surface is flat with less than 1 nm rms roughness when it was annealed in argon/hydrogen atmosphere. Then the surface forms nanofacets when it is annealed in pure air. This nanofaceting transition was completely reversible and reproducible. We investigated effects of CO adsorption on the voltammetric characteristics of both hydrogen-annealed and air-annealed surfaces. We found that C)-adsorption/desorption cycles in CO containing electrolyte solution result in considerable modification of blank cyclic voltammograms for the both surfaces. We attributed these differences to the electrochemical annealing of surface defects due to the increased mobility during the cycles.