A check valve comprises a valve body having inlet and outlet passages and a cavity in which a diaphragm operates. The inlet passage connects with the cavity through an upstream wall partly defining the cavity and the outlet passage connects with the cavity through a downstream wall partly defining the cavity. The diaphragm has an annular sheet metal part which is secured to one of the walls and carries a movable seal. The movable seal cooperates with a sealing element to provide a circular fluid seal. In a first embodiment, the inlet passage connects with the cavity through openings located inside the periphery of the circular fluid seal. In a second embodiment the inlet passage connects with the cavity through openings located outside the periphery of the circular fluid seal. The movable seal may be a sealing surface in which case the sealing element is a seal ring, or the movable seal may comprise a seal ring in which case the sealing element is a sealing surface.