The evolution of the surface structure in dodecanethiol self-assembled monolayer on Au(111) substrate has been studied with ultra high vacuum scanning tunneling microscopy at several temperatures. The structure of substrate Au(111) surface changed suddenly at a temperature of 110 degrees C. The enhanced mobility of the substrate gold atoms at this temperature is attributed to the desorption of the dodecanethiol molecules. (c) 2005 Elsevier B.V. All rights reserved.