An S-phase intermetallic compound (Al2CuMg) was synthesized in bulk form to characterize its, electrochemical behavior by conventional techniques. Large S-phase crystals, suitable for analysis, were formed in an Al-Cu-Mg ingot. The structure of Al2CuMg was verified by electron-backscatter Kikuchi patterns and X-ray diffraction. The composition of Al2CuMg was verified by electron-probe microchemical analysis. Open-circuit potential and potentiodynamic polarization measurements were conducted in aerated and deaerated 0.5 M NaCl solutions. Bulk S-phase electrodes exhibited steady-state open-circuit potentials ranging from -0.920 to -0.930 V-SCE, which did not vary significantly with changes in solution aeration. In aerated solutions, the phase dissolved vigorously at modest anodic polarizations, and no passive response was detected. Limiting cathodic current densities on the order of 1.0 mA/cm(2) were measured for small cathodic polarizations, probably due to the development of a porous, catalytic surface. Overall, the S phase supports rapid anodic and cathodic reaction kinetics, which contribute to the instability of the compound and subsequent Liberation of Cu. These results implicate the presence of S-phase intermetallic particles as a key contributor to the poor corrosion resistance of Al-Cu-Mg alloys.