An investigation has been made on corrosion of binary copper alloys in water at high temperature and steam under saturated pressure, 86 kg/cm², at 300°C for 250 hours, using an autoclave. Binary alloys containing zinc (10, 30, 40%), aluminium (1, 6, 10%), silicon (1, 3, 4%), manganese (1, 5, 9%), nickel (5, 8, 15%), or tin (2, 6, 12%) were used as rolled and annealed strip. After the corrosion test had been made, tensile test, microscopical examination and X-ray analysis of the corrosion product were performed and the following results were obtained. (1) Cu-Zn alloys suffer from layer type corrosion. (2) Cu-Al and Cu-Si alloys of α single phase suffer from intergranular corrosion. Cu-Al alloy of duplex phase suffers from selective oxidation on second phase. (3) Cu-Mn, Cu-Ni and Cu-Sn alloys are almost immune to corrosion but Cu-Mn and Cu-Ni alloys have a little susceptivility to intergranular corrosion.