This thesis is devoted to the study of aromatic and heteroaromatic thiol selfassembly on Au single crystal surfaces. A range of surface spectroscopic and electrochemical techniques have been used to investigate the formation of complex assemblies in terms of the types of surface intermediates formed, adsórbate orientation and the mechanism of adsorption from the gas and liquid phases. The self-assembly process is reviewed, with particular emphasis on aromatic self-assembled monolayers (SAMs). The applicability of a range of surface sensitive probes to the characterisation of such assemblies, differences between adsorption from the gas phase and the liquid phase and limitations with respect to ex situ techniques are discussed. Initial work focused on an electrochemical and X-ray photoelectron spectroscopy (XPS) investigation of the self-assembly of heteroaromatic thiols, 2- mercaptobenzothiazole and 2-mercaptobenzimidazole, in terms of their surface bonding and molecular orientation following adsorption from the liquid and vapour phases, with additional information on substrate contamination effects. The structural and behavioural complexities of the heteroaromatic thiols motivated us to revert to the simplest aromatic thiol, benzenethiol. The coverage dependent surface bonding, molecular orientation and growth mechanism of benzenethiol on A u ( lll) are discussed, highlighting differences in SAM composition and bonding configurations between adlayers prepared by gas phase and liquid phase adsorption. The influence of atomic steps on SAM formation compared to benzenethiol adsorption on an atomically flat surface is addressed. This study represents the first report of manipulation of the thiolate adlayer structure and molecular orientation by controlled addition of surface defects using Au(322), a stepped surface, as a substrate for self-assembly.