A gas-liquid reactor is described, in which a plunging jet issues into a confined downcomer column, entraining headspace gas as it impinges onto the surface of a receiving pool of the same liquid. The two-phase mixture flows down the confining column and then rises through a concentric annulus, before disengaging in a separation vessel. Such a design gives increased gas-liquid contact times compared to jets plunging into open pools. Another feature of the current design is that it is straightforward to recycle both the gas and liquid phases from the separation, using only a single pump, thus ensuring almost complete utilization of the gaseous feed. A hydraulic model of the reactor is presented and tested against experimental data over a range of liquid flow rates and nozzle sizes. The agreement between the predictions and the experimental data indicates that the model provides a good basis for the design of this type of gas-liquid reactor. It is also shown that downcomers with diameter greater than 24 mm appeared to have little effect on the gas entrainment rate, when nozzles of 5 to 12 mm diameter were used. Smaller downcomers significantly reduced the gas volumetric entrainment flow rate.