A detailed experimental study on the time-dependent flow behaviour of the liquid phase in bubble columns was performed to provide data for the validation of dynamic flow models. It focuses on the dynamics of circulation flows employing laser Doppler anemometry. In a flat bubble column the velocity time series in lateral and vertical direction are similar to undamped harmonic oscillations. Amplitude, frequency and phase angle describe them when superimposed turbulent fluctuations are removed by low-pass filtering. Lateral and vertical profiles of these properties prove the local character of the hydrodynamics. In a cylindrical bubble column the velocity time series are chaotic and not predictable. A broad range of low-frequency oscillations marks the unsteady flow behaviour. Nevertheless, regular flow patterns with periods of 20 s and more are sometimes observed. It is assumed that they are linked to the radial movement of the bubble hose in the sparger zone.