Abstract: An aeration/flotation reactor, of particular usefulness in the treatment of biomass-containing liquids, comprises a foam tube which terminates at the top in a collecting vessel and overlaps, or is inverted over, an inner rising tube which is preferably coaxial. The latter tube delimits an inlet chamber positioned, at the bottom of the foam tube, to receive liquid and finely divided gas bubbles. The lower end of the foam tube is enlarged, preferably into a funnel-shape, and runs into the lower region of an outer tube, the upper end of which is connected, via a return tube, to the inlet chamber below the axial rising tube. The outer tube, which is open at the top, is provided with a T-shaped liquid outlet having a dip connector, which extends below the liquid level, and with an outlet level immediately above the junction of the return tube, The upper end of the foam tube can be extended telescopically, thus permitting adaptation of the device to liquid/gas systems characterized by different foam properties.