Small bubbles with average diameters of 10 and 100 nm were spontaneously formed on the surface of Au(111) in contact with air-saturated water and ethanol-water 1/4 solution, respectively. Analyses of topographic images of the bubbles demonstrated that the contact line tension changed sign, from negative to positive, with a decrease in the bubble size. Despite the apparent stability and long lifetime of the bubbles, both of these bubble systems were found to be thermodynamically unstable, as estimated from the free energies of bubble formation by combination of their surface and line energies. (C) 2008 Elsevier B.V. All rights reserved.