A great deal of research has been done regarding bubble formation from submerged orifices in liquids under the force of gravity for the design of gas-liquid or gas-liquid-solid contacting equipment. On the other hand, little research has been done conceming bubble formation under reduced gravity conditions. For the basic design of the chemical process systems or life-support systems in space stations and on other planets, it is important to clarify the effects of various factors on the volume and shape of bubbles formed at submerged orifices or nozzles under reduced gravity conditions. In order to disperse adequately bubbles in liquids for mass transfer or chemical reaction processes at relatively low gas flow rates under reduced gravity, it is necessary to force bubbles to become detached from nozzles by external forces. In this study, the liquid flow was used as the external force on bubble formation. The aim of this study is to clarify the behavior of bubble formation in flowing liquids under reduced gravity conditions. We experimentally investigated the effects of gas flow rate, liquid flow velocity, and liquid flow direction (cocurrent, countercurrent or cross-current flow) on bubble formation for a period of 1.2 s under reduced gravity conditions that were produced in the 10 m drop tower at the Hokkaido National Industrial Research Institute at Sapporo in Hokkaido. In order to describe theoretically the bubble formation in flowing liquids under reduced gravity conditions, a revised non-spherical bubble formation model was proposed and the calculated results of the bubble volume were compared with the experimental ones. f: 1997 Elsevier Science Ltd