We developed a method of separating metallic single-walled carbon nanotubes (SWNTs) from semiconducting nanotubes (NTs) in suspension using a sodium hydroxide treatment and centrifugation. The semiconducting NTs remained suspended in the solvent while the metallic NTs precipitated out. This self-selective separation occurred because the metallic SWNTs bonded to hydroxyl groups due to strong surface hydrogen-bond interactions. We fabricated thin-film transistors based on random networks of semiconducting SWNTs on a glass substrate and obtained an on/off current ratio of 6.25 x 10(4). (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3236389] All rights reserved.