A new suggestion for the structure of the Au substrate underlying self-assembled monolayers (SAM) made of thiols is presented on the basis of density functional theory results for methylthiolate (SCH3) adsorption on Au(1 1 1). It is found that by introducing vacancy defects on the substrate, the adsorption of SCH, is stabilized by about 0.8 eV with respect to the perfect Au(1 1 1) surface. As this overcomes the vacancy formation energy (approximate to 0.6 eV), it net driving force exists leading to an adsorbate-induced reconstruction, that enhances adsorption at defected Au(1 1 1). A comparison of results at high and low SCH3, coverage provides further insight into which specific gold vacancy sites enhance the adsorption energy of the SCH3 molecule. (C) 2002 Published by Elsevier Science B.V.