In experiments with the jet bubble column, it was observed that the vertical jet in a jet bubble column can be unstable; that is, the jet does not always travel through the centre of the column, but under some conditions can attach to the wall of the column, causing an uneven distribution of the gas. This phenomenon is due to the Coanda effect, in which the presence of a wall causes one side of a let to experience a decrease in velocity, thus causing the entire jet to bend toward the wall. This effect has been known for many years for single-phase flow but appears not to have been studied for the two-phase gas-liquid system. Experiments show that the wall-attached jet does not remain in one place, but tends to move randomly about the perimeter of the column. Such an effect can have important consequences for mixing and mass transfer in the bubble column.