There is disclosed a diaphragm for a high pressure gas diaphragm valve. The diaphragm comprises a sealing body defining a sealing face for seating against a valve seat of the valve to close the valve; an outer flange for attaching the diaphragm to the valve, the outer flange defining a flange plane; and a deformable web that extends between the outer flange and the sealing body, the deformable web including a gusset that has at least one convolute portion. The gusset is deformable to allow the sealing body to move between a natural configuration in which the sealing body is on a first side of the flange plane, and a raised configuration in which at least part of the sealing body is on the opposing side of the flange plane, and wherein the at least one convolute portion biases the sealing body towards the natural configuration.