We have studied the structure and the electrochemical behavior of ethanethiol self-assembled monotayers (SAMs) on Ag(1 0 0) and Ag(1 1 1) in 0.1 M H2SO4, using in situ scanning tunneling microscopy (STM) and cyclic voltammetry. On Ag(1 0 0) an incommensurate structure with a next-neighbor distance of 0.44 nm was observed, whereas on Ag(1 1 1) a commensurate (root7 x root7)R19.1degrees structure with two different adsorption sites is found. The assembly process leads to a facetting of the surface, but no vacancy islands have been seen on Ag(1 0 0) and Ag(1 1 1). The electrochemical behavior of the SAMs on silver is compared with gold. (C) 2003 Elsevier B.V. All rights reserved.