The results are reported of an experimental study of the gas holdup, E-G, large bubble diameter, d(Lb), and large bubble rise velocity, V-Lb, in a 0.1 m wide, 0.02 m deep and 0.95 m high rectangular slurry bubble column operated at ambient temperature and pressure conditions. The superficial gas velocity U was varied in the range of 0-0.2 m/s, spanning both the homogeneous and heterogeneous flow regimes. Air was used as the gas phase. ne liquid phase used was C-9-C-11 paraffin oil containing varying volume fractions (epsilon(S) = 0, 0.05, 0.10, 0.15, 0.20 and 0.25) of porous catalyst (alumina catalyst support, 10 % < 10 mum; 50 % < 16 mum; 90 % < 39 mum). With increasing slurry concentrations, EG is significantly reduced due to enhanced bubble coalescence and for high slurry concentrations the "small" bubbles are significantly reduced in number. By the use of video imaging techniques, it was shown that the large bubble diameter is practically independent of the gas velocity for epsilon(S) > 0.05 and U > 0.1 m/s. The measured large bubble rise velocity VLb agrees with the predictions of a modified Davis-Taylor relationship.