An aeration tank, particularly for use in the treatment of sewage liquor, trade effluent and the like, comprises a chamber (6). generally oval in plan, to provide a continuous flow path F, inlet and outlets pipes (10, 12) being provided in conventional manner. Extending across the flow path is impeller means in the form of a rotor (14) on which paddles (16) are mounted. Downstream of the impeller and extending across the flow path is a deflector means afforded by a baffle (18). The baffle extends from a level adjacent to, preferably above, the surface of the liquor in the tank to a level below the surface of the liquor, and is inclined so as to cause liquor impelled by said impeller means, and air entrapped in the liquor, to be deflected towards the bottom of the tank. In this manner a high oxygen input may be obtained with a relatively low power input.