Abstract: A diaphragm pump is disclosed which includes flow channels (12;12a) within pressure chambers (9;9a), at least one of the walls defining the flow channels being in the form of a diaphragm wall (16,17; 16a,17a). A driven piston (A;B) displaces a liquid within the pressure chambers (9;9a) to cause cyclical changes in the volume of and pressure subsisting within the flow channels. Compensating pistons (S;T) are disposed generally opposite each of the driven pistons (A;B) and the compensating pistons are connected to a common hydraulic circuit such that the compensating pistons remain in position until the pressure in one of the pressure chambers (9;9a) exceeds a predetermined pressure, whereupon the compensating pistons (S;T) react in a manner tending to prevent further rise in the pressure in that chamber.