The adsorption of several molecules with a pyrrolidone ring (triacontane-pyrrolidone (NTP), 2-pyrrolidone (Py) and methyl-2-pyrrolidone (Mpy)) on an Au(111) surface has been studied by scanning tunnelling microscopy (STM) and by XPS, Self-assembled monolayers have been observed only for NTP and Py molecules, whereas STM images show a disordered Mpy overlayer. The XPS measurements on Py and Mpy monolayers reveal that the nitrogen atom of the pyrrolidone group is involved in different chemical bonds, depending on the molecule. Considering both STM and XPS data, models of molecule packing are proposed. Finally, tacking advantage of the different locations of the pyrrolidone function in these monolayers, we discuss the parameters that govern the presence of the pyrrolidone group in STM images, Copyright (C) 2000 John Wiley & Sons, Ltd.