The results are presented for the following three experiments on the upward flow of airwater mixtures in a vertical tube having an inside diameter of 51 mm. (a) Visual observation of the flow patterns. (b) Measurement of the static pressures along the tube. From these results, the volume fraction of air in the tube and the relative velocity of air were calculated. A tentative correlation is presented about the drag coefficient of the force acting between air and water owing to the relative velocity. (c) Measurement of the tube wall temperature under a constant heat rate. The flow rates include the range of 200 to 5000 kg/h for water and 0 to 45 kg/h for air. It seems that the obstacles in tubes have particular effects on the flow of air-water mixtures. Then, some experiments were also performed to make qualitatively clear the effects of obstacles inserted into the vertical tube.