Abstract: An improved apparatus is described wherein a semi-permeable membrane is supported against a convex surface and wherein the likelihood of damage to such membrane during mounting and use is minimized. In the apparatus the membrane is stretched over the convex surface of a first member in a manner allowing, during assembly of the apparatus, any fluid located between the membrane and the convex surface to escape therefrom around the edges of the membrane. The membrane is clamped between the convex surface of said first member and a complemental concave surface of a second member such that the membrane is held in stretched relationship against and in close contact with the convex surface. Means is provided to form a fluid-tight seal between said membrane and said second member.In a preferred embodiment, the membrane is used in a polarographic electrode apparatus used to detect specific components of a fluid sample.