The roles of two model additives, bis(3-sulfopropyl) disulfide (SPS) and Janus Green B (JGB), in the deposition of copper from an acid-copper sulfate electrolyte containing polyethylene glycol (PEG) and Cl- were studied by deposition on microprofiled electrodes, scanning electron microscopy (SEM), and transmission electron microscopy(TEM). It was found that leveling occurs only when all four additives are present, suggesting that additive-additive interactions are important to the leveling mechanism. Moreover, an optimal flux of the active leveling agent exists, an effect that may be explained by the classical diffusion-adsorption theory of leveling. SEM and TEM micrographs show that the additive SPS removes the columnar structure of the deposit and effects micron-sized, unoriented grains provided PEG and Cl- are present; the subsequent addition of JGB decreases the grain size of the film significantly.