An equation due to Elmore relating the time required to establish polishing conditions to the current applied has been tested with a copper anode in phosphoric acid. The dependence of a constant in Elmore's equation on anode area, phosphoric acid concentration, copper solubility, viscosity, and copper concentration has been investigated. The results show that the process is diffusion‐controlled but that Elmore's hypothesis, that the onset of polishing coincides with the attainment of the solubility limit of copper in the electrolyte, is incorrect, since additions of copper to the electrolyte were found to have no effect on the constant, except insofar as they modified the viscosity. Two possible explanations of these results are advanced, based on the view that the controlling process is depletion of the anode layer with respect to either hydrogen ions or to those ions or molecules with which the copper ions are combined in the solution. It is shown how the latter interpretation can explain the principal features of electropolishing.