Hydrodynamic characteristics of bubble columns are studied by considering both the local/bubble-scale and global/column-scale properties of the system, with specific emphasis on their dependency on operating pressure and temperature. The local-scale properties include single-bubble characteristics and interactive dynamics involving bubble coalescence and breakup while the global-scale properties include gas holdup. Experiments are conducted in a bubble column with a multiorifice ring sparger for pressures as high as 20 MPa and temperatures ranging from 27 to 78 degrees C. Density, viscosity, and surface tension, the liquid phase properties, are measured in situ by using hydrostatic weighing, falling-ball and emerging-bubble techniques, respectively. The bubble behavior is examined through direct-flow visualization. Pressure and temperature affect the bubble rise velocity mainly by varying physical properties of the fluids. Effects of pressure and temperature on the rates of bubble formation, coalescence and breakup, and the initial bubble size and maximum stable bubble size are illustrated. Elevating pressure and/or temperature increases gas holdup; the increasing rate depends on the operating conditions through their effects on the fluid physical properties.