Volumetric gas-liquid mass-transfer coefficients are investigated in bubble columns under high-pressure and moderate-temperature conditions by utilizing an oxygen desorption method. The oxygen concentration in the liquid phase, water or Paratherm NF heat-transfer fluid, is monitored with a high-pressure optical fiber oxygen probe. The study covers operating conditions up to pressures of 4.24 MPa and up to temperatures of 92 degreesC. The superficial gas and liquid velocities vary up to 40 and 0.89 cm/s, respectively. Experimental results show that system pressure, temperature, gas and liquid velocities, liquid properties, and column dimensions are major factors affecting mass transfer. The mass-transfer coefficient increases with both pressure and temperature. Both gas and liquid velocities improve mass transfer due to higher turbulence at high-velocity conditions. Liquid properties and column dimensions also have significant effects on mass transfer. The effect of liquid velocity on k(1)a is mainly due to the change in k(1), while other variables affect k1a mainly through the change in gas holdup, which directly affect the interfacial area, a. A consideration of the dispersion term on the determination of k(1)a gives results similar to those of the continuous stirred tank reactor model, which was used to determine k(1)a in this study.