The aim of this study is to investigate the structure of upwards air-water flow and its variation as the bubble-to-slug transition is approached, with a view to understanding the mechanism of the transition. The development of bubble size with column height is measured with a double needle resistivity probe, and of void fraction waves with an Impedance Void Fraction Meter (IVFM) with quadrant electrodes in a 150 mm diameter column. The results are compared with some experimental data gathered in a 28.9 mm column. Traditional slug flow does not exist in the 150 mm diameter column; instead there is a gradual transition to churn flow. Possible instabilities of void fraction waves are detected during the transition from bubble to churn at constant liquid rate, but not at constant gas rate. In the 28.9 mm column at constant liquid rate, an abrupt transition from bubble-to-slug flow occurs, with strong indications of associated void fraction wave instabilities.