The influence of pressure, liquid viscosity, and gas velocity on the gas holdup and specific gas−liquid interfacial area in a bubble column reactor has been studied. The 18.7 L reactor had an inner diameter of 15.6 cm with a dispersion height set equal to 3 times the diameter and was operated at pressures between 0.1 and 6.6 MPa. By means of the chemically enhanced absorption of CO2 in diethanolamine, the gas−liquid interfacial area in the reactor has been determined. The viscosity has been changed in the range from 1 to 9.4 mPa·s by adding ethylene glycol to the mixture. It is determined that pressure has a small effect on the gas holdup in pure water, whereas it shows a pronounced effect for the more viscous liquids. This can be attributed to the influence of the increased pressure on the flow regime transition. For the most viscous liquid all interfacial area data were obtained in the fully heterogeneous regime. Here the interfacial area increased with increasing pressure and was moderately affected by the gas velocity. For the less viscous liquids both pressure and gas velocity affect the interfacial area; this influence depends on the flow regime. Therefore, the state of the flow regime has an important impact on the mode in which the operating parameters affect the interfacial area.