When a down-flowing liquid jet impinges onto the free surface of a pool of the same liquid, the jet entrains the surrounding head space gas. The plunging jet creates a dispersion of small bubbles in the receiving pool, establishing a large interfacial area. This two-phase region may be exploited for oxygen mass transfer in the activated sludge treatment of high biological oxygen demand (BOD) wastes. Use of a plunging jet oxygenator within a down-flow bubble column can lead to almost total absorption of the supplied oxygen with a low energy input. A laboratory scale study has been conducted on the mechanism of gas entrainment by a plunging jet. The effect of jet properties, including turbulence intensity, on the minimum entrainment velocity and the bubble penetration depth has been investigated. These measurements allow preliminary predictions of the optimum operating conditions for industrial scale treatment processes.