In industrial practice bubble column reactors are often operated at elevated pressures. Despite numerous studies on hydrodynamics in bubble columns, the number of experimental studies performed under high pressure is limited. This study reports the combined effect of high liquid viscosity, column diameter and operating pressure on the total gas hold-up, which has not been published previously in the open literature. The experiments presented in this study are performed under pressure (0.1 to 1 MPa) with Newtonian viscous liquid media up to n(L) = 0.55 Pa. It was already shown in literature that increasing the viscosity of the liquid results in a pronounced decrease in the total gas hold-up (Urseanu, Scaling up bubble column reactors, Ph.D. Dissertation in Chemical Engineering, University of Amsterdam, Amsterdam, 2000; Krishna et al., A scale up strategy for bubble column slurry reactors, Catalysis Today 66 (2001a) 199). The same authors found that total gas hold-up reduces with increasing column size, both for low- and high-viscosity liquids. Increasing the operation pressure leads to a considerable increase in the total gas hold-up. In this study it was found that the effect of the pressure gradually disappears as the liquid viscosity increases. (C) 2003 Elsevier Science Ltd. All rights reserved.