Abstract Copper deposition in a plating bath known to fill narrow trenches was studied on a rotating disc electrode by impedance measurements. The chemistry of this bath contained, in addition to sulphuric acid, copper sulphate and chloride ions, polyethylene glycol (PEG) as an inhibitor and 3-mercapto-1-propanesulfonate, sodium salt (MPSA) as an accelerator. The experimental results were compared with a model taking into account these organic additives. A competitive adsorption of two complexes: PEG–Cl–CuI and CuISPS formed from Cu+ and the additives, is proposed. A good agreement was shown between this model and the experiments on fresh plating bathes.