The electrochemical behavior of copper in aqueous solutions containing phosphoric acid (orthophosphoric acid) and phosphate salts is important in a variety of industrial applications. This study presents aqueous solubility diagrams and potential-pH diagrams for a copper-phosphoric-acid-water system at a wide range of total dissolved copper, {Cu-T}, and phosphoric acid activities, {OPT}. Cu-3(PO4)(2) and Cu(PO4)(2)center dot 3H(2)O were used as the cupric-phosphate solid species in the constructions of the diagrams. The stability domains of these two solids were similar, but Cu(PO4)(2)center dot 3H(2)O had a relatively narrower predominance region. The diagrams showed that a higher amount of {Cu-T} is required to form the copper-phosphate solids when {OPT} is increased. For the combinations of lower {Cu-T} and higher {OPT} values, phosphoric acid is expected to increase the solubility range of copper through the formation of aqueous copper-phosphate complexes. Increasing the temperature broadens the stability domain of Cu-3(PO4)(2). The aqueous solubility diagram constructed for copper electropolishing rules out the formation of any anodic solid films on the copper electrode during electropolishing in 85 wt % phosphoric acid. The predictions of the diagrams agree with the experimental data from the literature. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3215996] All rights reserved.