Electropolishing of copper in phosphoric acid was investigated through mathematical modeling, using a model based on thermodynamics and transport theory. The electropolishing process consists of two distinct regimes: the copper-accumulation regime and the salt-film-growth regime. The concentrations of cupric ion and a certain anion at the anode surface increase in the first regime as a result of transport limitations, until the saturation level of a certain cupric salt is reached. In the second regime, the salt film forms and grows on the anode surface because of thermodynamic requirements. The model can be used to calculate the onset point of film formation, the growth rate of the film, and the thickness of the film.