The superwetting or dewetting properties of a silicon nanowire (SiNW) surface are studied. The SiNW arrays were grown vertically on SiO2 or Si substrates by a vapor-liquid-solid process. After a critical length or roughness, the nanowire surface exhibits superhydrophilic property with a contact angle close to 0 degrees. However, the superhydrophilicity of the surface can be transformed into superhydrophobicity by a vacuum treatment, which is a low pressure diffusion process by the vapor from the polydimethylsiloxane material. In addition, the superhydrophobicity of the surface can be readily reversed back to superhydrophilicity by 600 degrees C anneal with N-2 gas flow. The reverse nature of surface wettability can be repeated for many cycles by alternating vacuum with thermal treatment. Physical explanation together with analytical measurements are provided and discussed. The findings have opened a door to completely control the superhydrophilicity or superhydrophobicity of a surface with a very simple and cheap method. (C) 2010 The Electrochemical Society. [DOI:10.1149/1.3464765] All rights reserved.