The effect of quaternary ammonium salts on the electrodeposition of Cu from acidified sulfate solutions on Au(lll) single-crystal electrodes was studied with in situ atomic force microscopy, electrochemical, and X-ray diffraction techniques. Cyclic voltammetry showed that hexadecyltrimethylammonium bromide [CTA(Br-)] adsorbs strongly on the Au substrate, thus inhibiting Cu deposition both in the underpotential deposited (upd) and bulk deposition region. Cu electrodeposits formed in the presence of CTA(Br-) were found to consist of regularly shaped triangular crystallites, whose orientation was influenced by the topographical characteristics of the Au substrate. The distinctive morphology was attributed to the combined presence of CTA(+) and Br-. The Cu crystallites were observed to form in a layer-by-layer fashion, and the frequency of layer formation and lateral growth rate were found to depend on the applied overpotential. A one-dimensional transport model was used in order to assess the adatom concentration on the growing crystallites and to provide qualitative insight into the deposit growth habit.