Interfacial areas and gas hold-ups have been determined at pressures up to 8.0 MPa in a mechanically agitated gas—liquid reactor and a bubble column with a diameter of 81 mm for superficial gas velocites between 1 and 5 and 1 and 10 cm/s, respectively. The interfacial areas have been determined by the chemical method using the model reaction between CO2 and aqueous diethanolamine (DEA). Contrary to earlier reported results on interfacial areas in a mechanically agitated reactor at pressures up to 1.7 MPa, a positive influence of pressure on the interfacial areas has been observed for higher pressures and higher superficial gas velocities. The product of the gas density G and the superficial gas velocity at the orifice υG, or was found to be an important parameter for the manifestation of the pressure effect. For values of GυG, or larger than 200 kg/m2 s the interfacial areas increase with increasing reactor pressure. Below this value of 200 kg/m2 s no influence of pressure could be observed. The gas hold-ups in the bubble column in water as well as in an aqueous solution of DEA with antifoam increase with increasing pressure. This pressure effect on the gas hold-up in bubble columns originates from the formation of smaller bubbles at the gas distributor. The relative increase in the gas hold-ups is smaller in water and also if a porous plate instead of a perforated plate is used as gas distributor. The differences in the magnitude of the pressure effect are caused by differences in the coalescence behaviour of the gas bubbles in both liquids and by differences in the bubble formation process at the two types of gas distributors, respectively. The interfacial areas in the bubble column also increase with increasing pressure. The relative increase in the interfacial areas aP/aatm with increasing pressure may be as large as 200% for a pressure increase from P = 0.15 to 8.0 MPa, depending on the type of gas distribution and the superficial gas velocity used.