In this study a gamma-ray densitometry was used to measure radial and axial volume fraction distributions in a 0.21 m diameter bubble column, and flow regime transitions in a 0.05 m and 0.21 m diameter bubble columns. Experiments were conducted with two- and three-phase systems (nitrogen-Fischer-Tropsch derived waxes-iron oxide or silica particles) in both batch and continuous modes of operation at 265 degrees C and ambient pressure. Average radial gas holdups in the large diameter column showed the existence of a maximum at the center of the column, in both two- and three-phase systems. The maximum was more pronounced at higher superficial gas velocities (churn-turbulent flow regime). The axial and average gas holdup measurements in two-phase systems were in good agreement with results obtained by a conventional method (pressure measurements along the column height). An independent treatment of all three phases did not yield satisfactory results, due to large impact of small experimental errors on calculated holdups. Alternative methods based on measured values of solids concentration in the slurry yielded satisfactory results. Instantaneous signals from the nuclear density gauges were used successfully to determine transitions from homogeneous flow regime to slug flow (small diameter column) or churn-turbulent flow regime (large diameter column) in two-phase systems.