The reaction mechanism of Cu dissolution in 0.1 M Na2SO4 acidified to pH 1.5 was investigated. For this purpose, in addition to the steady-state polarization curve and electrode impedance measurements, two new ac techniques were used. First, electrogravimetric measurements that determine the mass change of an electrode by using a quartz crystal microbalance (QCM). Secondly, electrocoulometric measurements which evaluate the flux of species leaving the disk by means of a rotating ring-disk electrode (rrde). It was established experimentally that Cu dissolves through a monovalent species at low current densities and a divalent one at high current densities. Electrogravimetric measurements under an ac signal showed further the existence of an adsorbed intermediate. The electrocoulometric method corroborates this conclusion and the differential charge stored at the electrode surface (ΔQ/ΔE) was found to be equal to a few millifarads per square centimetre. A reaction mechanism was proposed involving two dissolution paths, one forming a monovalent and the other one a divalent Cu ion. Numerical simulation on the basis of this model showed fairly good agreement with experimental data.