The hydrodynamics of bubble columns with concentrated slurries of paraffin oil (density, rho(L) = 790 kg/m(3); viscosity, mu(L) = 0.0029 Pa.s; surface tension, sigma = 0.028 N.m(1)) containing silica particles (mean particle diameter d(p) = 38 mu m) has been studied in columns of three different diameters, 0.1, 0.19 and 0.38 m. With increasing particle concentration, the total gas holdup decreases significantly. This decrease is primarily caused by the destruction of the small bubble population. The hold-up of large bubbles is practically independent of the slurry concentration. The measured gas hold-up with the 36% v paraffin oil slurry shows remarkable agreement with the corresponding data obtained with Tellus oil (rho(L) = 862 kg/m(3); mu(L) = 0.075 Pa.s; sigma = 0.028 N.m(-1)) as the liquid phase. Dynamic gas disengagement experiments confirm that the gas dispersion in Tellus oil also consists predominantly of large bubbles. The large bubble hold-up is found to decrease significantly with increasing column diameter. A model is developed for estimation of the large bubble gas hold-up by introduction of an wake-acceleration factor into the Davies-Taylor-Collins relation (Collins, 1967), describing the influence of the column diameter on the rise velocity of an isolated spherical cap bubble.