We have investigated L-cysteine adsorbed on Ag(1 1 1) electrodes Under different conditions. We have employed experimental and theoretical approaches to obtain a better understanding of the adsorbed layer. An estimation of the coverage front charge measurements and the second harmonic response shows C-3v symmetry for the interface indicating a (root 3 x root 3)R degrees 30 overlayer. The theoretical calculations show a variety of different structures with local adsorption energy minima. Particularly, under special initial conditions, zwitterionic structures adsorbed at different sites have been found. This can account for the multiplicity of redox processes observed experimentally below the potential of zero charge. The presence of an external field produces the stabilization of the zwitterion by interaction of the amino/carboxylic groups with the Substrate. (c) 2008 Elsevier Ltd. All rights reserved.