The kinetics of the dissolution of the SAM formed by 6-mercaptopurine (6MP) on Au(1 11) and Hg electrodes in alkaline media (0.1 M KOH) were studied by single step chronoamperometry. The shape of the chronoamperometric curves shows a current maximum that depends on the magnitude of the potential step. These curves are typical of a nucleation and growth mechanism. Quantitative evaluation of experimental transients was performed by a non-linear fitting procedure. The analysis of the kinetic parameters shows that the dissolution process could be characterized as a 2D-nucleation of etching centres (holes) following an exponential law in combination with a surface diffusion controlled growth. Our results show that the slower kinetics observed when the monolayer is formed in Au(111) in comparison with Hg, are mainly due to differences in the rate of nucleation that is more than one order of magnitude higher in the latter substrate. (C) 2004 Elsevier B.V. All rights reserved.