Bubble columns are widely used as chemical reactors. The liquid phase is well mixed due to a liquid circulation present in practically all technical applications. The liquid circulation is shown experimentally (Lippert, 1982) to contain several circulation cells. Each cell is about one bubble column diameter high and undergoes fluctuations in position and circulation velocity. The liquid circulation is suppressed if the gas phase is distributed very evenly over the cross-sectional area of the bubble column as shown experimentally by Molerus and Kurtin (1985, Chem. Engng Sci. 40, 647--652). Thus, the liquid circulation is a flow instability caused by a disturbance of gas distribution. The onset of the liquid circulation is investigated in the present paper by applying an analytical method. The flow field of several consecutive circulation cells is obtained applying a numerical method, which is specially adapted for this kind of flow instability. Our focus is on understanding the mechanism of the circulation cells. Thus, all terms of smaller orders of magnitudes are neglected in the mass and momentum balances for both phases. The main terms, which cause the flow instability, are extensively discussed. We assumed a small but stationary disturbance of gas distribution in order to simplify the calculations. The general trend in two-phase modeling, even though there are some remarkable exceptions, is to implement more and more forces and higher sophisticated turbulence models. But circulation cells are an example that some flow phenomena may not be described following strictly that way. Basically, a very high spatial resolution and a very stable numerical method are required in order to predict circulation cells.