A comparative study of the influence of a new additive (IT-85), based on ethoxyacetic alcohol and triethyl-benzyl-ammonium chloride, and thiourea on copper electrodeposition from acidic sulfate solutions has been performed in order to obtain information about the kinetics of the cathodic process. In spite of their different chemical nature, both additives were found to be efficient as leveling agents, leading to fine-grained cathodic deposits. The study was based on scanning electron microscopy and X-ray dispersive analysis of the Cu deposits, coupled with an electrochemical investigation using cyclic voltammetry (CV) and steady-state polarization measurements at a rotating disc electrode. CV results and kinetic parameters obtained from Tafel plots led to the conclusion that both additives have a pronounced inhibiting effect on Cu2+ discharge, the strongest inhibition being observed for thiourea. An induction period related to a slow nucleation, increasing with additive concentration, was clearly put on evidence on the polarization curves, for both additives.