This paper reports on the mode of action of two different organic additives—gelatine and thiourea—during the electrorefining of copper from acid copper sulfate solutions. Gelatine increases the cathode current efficiency and produces smoother deposits up to a certain level of concentration, beyond which, however, these effects are diminished by the steric hinderance of bulky molecular entities adsorbed to the electrode surface. Thiourea decreases the cathode current efficiency when present at concentrations around 5 mg/1. Nonetheless, it improves deposit quality. In higher concentrations, thiourea increases the cathodic current efficiency but also promotes nodule formation and rough deposits. The degradation and/or hydrolysis of both additives and the various interactions with the electrode surface and with cupric ions in solution are also examined.