Cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM) have been used for studying the binding of protons and cadmium(II) ions by 8-mercaptooctanoic acid monolayer that was assembled on an Au surface, Binding of the cations by the monolayer affected the kinetics of electron transfer to negatively charged species. i.e.. IrCl63 , in the solution. Increasing the pH of the solution causes the CV of IrCl63 to become less reversible and the feedback current of the SECM to decrease its the distance between the microelectrode and the surface decreases. On the other hand. addition of Cd2+ to the solution at pH 6.8 increases the CV reversibility and the feedback current of the SECM as a result of neutralizing the negative charge of the monolayer by the metallic ions. A model based on the Frumkin correction Was used to account for these findings and for extracting the heterogeneous association constant of Cd2+ by the monolayer. (c) 2005 Elsevier B.V. All rights reserved.