The relative concentration of hydrogen ion (H+) as a function of sulfuric acid (H2SO4) concentration (calculated using Pitzer's model) and the electrochemical processes by which irregular copper deposits are formed were correlated. Irregular deposits are formed by potentiostatic electrodeposition at a high overpotential where the hydrogen evolution reaction occurs parallel to copper electrodeposition. Two sets of acid sulfate solutions were analyzed. In one set of experiments, the concentration of CuSO4 was constant while the concentration of H2SO4 was varied. The other set of experiments was performed with a constant concentration of H2SO4 and different concentrations of CuSO4. Then, the volumes of the evolved hydrogen (calculated as the average current efficiencies of hydrogen evolution) and the morphologies of copper deposits, characterized by the SEM technique, obtained for the same ratio of CuSO4/H2SO4 were mutually compared and discussed in terms of the relative concentrations of hydrogen ions (H+) as a function of the H2SO4 concentration. Good agreement between the ionic equilibrium in the CuSO4-H2SO4-H2O system and the results of the electrochemical processes was obtained. In this way, it was shown how this ionic equilibrium can be applied to predict and analyze the solution composition in electrolytic copper deposition processes. (C) 2008 Elsevier Ltd. All rights reserved.