The holdups of small and large gas bubbles, bubble size distribution and the Sauter-mean bubble diameter were measured for N-2 and He in isoparaffinic organic liquid mixture (Isopar-M) in the absence and presence of Alumina powder under various pressures (0.67-3 MPa), temperatures (300-473 K), superficial gas velocities (0.07-0.39 m/s), and solid concentrations (0-20 vol.%) in a large-scale bubble column and slurry bubble column reactor (SBCR) (0.29 m diameter, 3 in height). The gas holdup was measured using the manometric method and the bubble size distribution, and Sauter-mean bubble diameter were obtained using the dynamic gas disengagement (DGD) technique and the photographic method. The experimental data showed that the total gas holdup increased with pressure and superficial gas velocity due to the increase of gas momentum which shifted the bubble size distribution towards smaller gas bubbles. The total gas holdup was also found to increase with temperature due to the decrease of liquid viscosity and surface tension. Increasing the solid concentration, on the other hand, resulted in a significant decrease of the total gas holdup and significantly increased the Sauter-mean bubble diameter. The online monitoring of the swarm using the high-speed camera showed a decrease of the froth stability in the reactor with increasing solid concentration and temperature which were responsible for the decrease of the total gas holdup. (c) 2006 Elsevier B.V. All rights reserved.