We present an experimental study of two-dimensional bubble plumes confined in a tank. The plumes undergo periodic oscillations. The non-stationary behaviour of the flow was studied with optical fibre probes and video camera measurements. Local void fraction and instantaneous positioning of the plume fronts were measured. For low gas flow rates, both fronts move in phase, and the geometrical centre of the plume moves in a globally periodic manner which was shown to be a progressive wave. A lot of flow configurations were observed for different gas flow rates (with or without strong interactions with the walls). A dominant periodic motion was observed even for high gas flow rates. Dependency of the oscillation frequency on the gas flow rate was measured and compared to other results from the literature. In the range of our experiments, it was observed that the water depth did not affect the structure of the plume.