The cold-seal pressure vessel (CSPV) is a commonly-used apparatus for hydrothermal experiments performed at crustal temperature (T)-pressure (P) conditions. The redox states of the samples in CSPV hydrothermal experiments are normally controlled by the pressure medium, by using oxygen buffers, or with the Shaw membrane technique. Although these techniques have been widely used, the actual redox states of the samples are not necessarily the expected values and need to be quantitatively measured. In this study, by using CoPd, NiPd, and (NiMn)O redox sensors, quantitative redox measurements have been conducted in H₂O-pressurized Ni-superalloy (Inconel 713 LC) CSPVs at 700-800 degrees C and 200 MPa. Oxygen fugacities (fO₂) of the H₂O pressure medium in three CSPVs used in this study were measured, and their log fO₂ values were - 13.11 +/- 0.06 (Ni-NiO (NNO) + 0.7), - 13.05 +/- 0.06 (NNO + 0.8) and - 12.68 +/- 0.07 (NNO + 1.1), comparable with those reported previously. By placing an open capsule containing Ni powder below the sample capsule, the fO₂ of the H₂O pressure medium can be adjusted to reach that of the NNO buffer (NNO to NNO + 0.2). For the MnO-Mn₃O₄, Ni-NiO, and Co-CoO oxygen buffers, a steady-state fO₂ was always achieved for each buffer at a fixed P-T condition. Measured log fO₂ values for the Ni-NiO and Co-CoO buffers are in good agreement with theoretical values calculated from thermodynamic data and also with previous results obtained from vessels pressurized with Ar. The log fO₂ values determined for the MnO-Mn₃O₄ buffer, about 0.3 log unit lower than the theoretical values, are consistent with previous values obtained from vessels pressurized with Ar. These results suggest that the use of H₂O pressure medium would not perturb the equilibrium redox states of the oxygen buffers. In addition, we developed a miniature sensor that is small enough to be loaded together with other sample material, allowing the monitoring of fO₂ in actual experiments.