In this paper we report a study of the electrodeposition of Cu from an acidic sulphate solution containing cetyltrimethylammonium bromide (CTAB). This investigation is based on cyclic voltammetry, in situ surface-enhanced Raman spectroscopy (SERS), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). Adsorption of CTAB over a wide range of cathodic potentials was proved spectroelectrochemically, encompassing both conditions where compact and loose Cu grows (up to -0.4 and between -0.4 and -1 V vs. Ag/AgCl, respectively). Furthermore, we found that CTAB tends to react cathodically, undergoing the loss of the aliphatic tail at lower cathodic potentials and the formation of an allylic product at higher polarisations. CTAB deeply impacts the Cu growth mode: ordered ridges of compact Cu crystallites form at low cathodic potentials-where the electrodeposition process is strongly inhibited by CTAB adsorption-and nanoparticles grow under hydrogen-evolution conditions.