Plating baths containing a mixture of leveling agents, where the mixture includes a first level agent having a first diffusion coefficient and a second leveling agent having a second diffusion coefficient, are provided. Such plating baths deposit a metal layer, particularly a copper layer, that is substantially planar across a range of electrolyte concentrations. Methods of depositing metal layers using such plating baths are also disclosed. These baths and methods are useful for providing a planar layer of copper on a substrate having small apertures, such as an electronic device.