Displacement of the suppressor polyethylene glycol (PEG) by the accelerant 3-mercapto-1-propanesulfonate (MPS) during copper electrodeposition was studied by a chronoamperometric addition technique. Displacement of PEG occurs through a process of MPS domain nucleation, mass-transfer limited domain expansion and an asymptotic approach to equilibrium between MPS in solution and that on the surface. An isotherm is proposed to account for the equilibrium coverage over a wide range of MPS concentrations, and the nearest neighbor separation between adsorbed MPS molecules during deposition is estimated. (c) The Author(s) 2014. Published by ECS. All rights reserved.