Free-jet turbines working under backpressure conditions represent an economical alternative to conventional hydro-electric plant configurations. However, the air introduced into the tailwater generates an air/water mixture. Its reaction to a rising ambient pressure is at present being studied at the above Institute. This report deals with the effect an increase in ambient pressure has on the volume and consistency of the two-phase mixture and the rise velocity of air bubble swarms. In addition, a test set-up is described which was used to study the physical reaction of the water/air mixture to a change in ambient pressure conditions. The results and their effects on the configuration of free-jet turbines working under backpressure are discussed.