New experimental data on local two-phase hydrodynamic behaviour in bubble columns are presented. Glass fibre techniques and pressure fluctuation analysis were applied in a 15.2 and in a 23.4 cm bubble column, both operated at aspect ratio 4. Measurements were performed at gas superficial velocities up to 8 cm s(-1). Various signal correlation, time windowing and noise analysis techniques were used in data analysis. The glass fibre measurements showed a clear time-dependent behaviour of the flow. The pressure fluctuation measurements support this view. The presence of one large-scale overall liquid circulation rather than of distinct circulation cells in the column was observed. The influence of the distributor plate on the flow in the column is discussed. These observations lead to a concept which treats a bubble column as a non-steady system.