Abstract The corrosion behavior of Inconel 625 in supercritical water was investigated under high pressure (30–100MPa at 400°C) with the addition of acids (5mmol/kg CH3COOH or 0.5mmol/kg HNO3) or O2 (2.5–250mmol/kg). Ni and Cr ions were the main dissolved metal ions in the effluent. The Ni ion concentration showed no clear dependence on the O2 concentration or pressure in the experiment with O2; the concentration was lower than 0.05ppm. The concentration increased with pressure up to 0.09ppm and 3ppm (at 100MPa) in the experiment with CH3COOH and HNO3, respectively. The Cr ion concentration increased with the O2 concentration and pressure; the maximum value of the concentration was about 0.5ppm. The Cr ion concentration was lower than 0.01ppm in the experiment with CH3COOH, while the concentration was considerably higher in the experiment with HNO3: the concentration increased with pressure up to 0.87ppm at 100MPa. The effect of pressure on the corrosion behavior of Inconel 625 was discussed using potential-pH diagrams, metal oxide solubilities, pH, and equilibrium of ionic reactions. Pressure dependence of the metal ion concentrations was analyzed using a model with water density as a parameter; the log–log plots revealed a linear relationship.