Abstract: A double membrane pump comprising a central housing (1), two coaxial product chambers (6) in the central housing, suction and pressure valves for each product chamber, two membranes (7) sealing off the product chambers externally from coaxial pressure fluid chambers (8), piston rods (13) guided in external seals, a U-shaped external crosspiece (17) connecting the piston rods of the membranes and a control block for reciprocally pressurising the pressure fluid chambers with pumping medium and comprising a slide valve (20) actuated by the movement of the membranes. The pump provides a very short path for the pumping flow, with only two changes in direction, and only static seals. It can be made wholly of plastics materials and is particularly suitable for high-purity products in the semiconductor industry, in biotechnology and in the pharmaceutical, cosmetic and foodstuffs industries.