Rates of electropolishing of the outer surface of horizontal and vertical rectangular copper ducts were studied by measuring the limiting current of the anodic dissolution of these ducts in phosphoric acid under natural convection conditions. Phosphoric acid concentration and duct dimensions were varied to provide Sc Gr range of 1.46 x 10(9)-1.42 x 10(11). Under these conditions rates of polishing of vertical ducts were found to fit the equation Sh = 1.05 (Sc Gr)(0.27) while rates of polishing of horizontal ducts were found to fit the equation Sh = 0.7 (So Gr)(0.233) Sc-0.1. The present study recommends the use of the vertical position in practice owing to the uniformity of the hydrodynamic conditions at the four vertical planes which give rise to a uniform degree of polishing all over the duct.