Abstract: A one-way valve (10) is disclosed which comprises a body portion (12) defining a passage through which fluid is able to pass during use, a support member (22) disposed in the passage, a seat member (28) disposed in the passage and provided with at least one aperture (32), a shaft (24) extending between the support member (22) and the seat member (28) and a membrane (26) disposed on the shaft (24) such that the membrane (26) is rotatable relative to the shaft (24). The seat member (28) cooperates with the membrane (26) such that a first predetermined pressure differential across the membrane (26) causes the membrane to cover the at least one aperture (32) and thereby prevent fluid flow past the seat member (28), and such that a second predetermined pressure differential of opposite polarity to the first predetermined pressure differential causes the membrane (26) to not cover the at least one aperture (32) and thereby permit fluid flow past the seat member (28).