In situ electrochemical scanning tunneling microscopy was used to study the coarsening of platinum nanocrystals under potentiostating and the dissolution and coarsening of platinum nanocrystals through potential cycling. A rapid coarsening of platinum nanocrystals was observed for potentiostating in the double-layer region. No obvious crystal size changes were observed for potentiostating in the platinum oxidation-reduction region and the hydrogen-adsorption proton-oxidation region. The coarsening is proposed as a variant of Ostwald ripening, whereby the dominant process is potential-induced surface-diffusion transport. Additionally, potential cycling causes not only surface platinum dissolution as a result of the platinum redox reaction. but also platinum nanocrystal coarsening. Details of the platinum nanocrystal restructuring are described. and the possible influences of these phenomena on fuel cell operation are discussed. (C) 2008 The Electrochemical Society.