In the waste water treatment industry, a novel gas sparger based on flexible membranes has been used for the last 10 years. The objective of the present work is to study the bubble formation generated from a flexible orifice (membrane). Firstly, the membranes are characterised with regard to their properties: wetting critical surface tension, expanding hole diameter, orifice coefficients, flexibility, critical and elastic pressures. The bubble formation phenomenon in an inviscid liquid at rest is studied experimentally for different membranes and gas flow rates. The variation in the bubble diameter, the bubble centre of gravity and the bubble spread on the membrane are determined as a function of time. An analytic model is proposed to describe the bubble growth and its detachment at a flexible orifice. This theoretical approach, developed by Teresaka and Tsuge (J. Chem. Eng. Jpn. 23 (1990) 160) for rigid orifices, is adapted to take into account the membrane features (elastic behaviour and wettability). The predicted bubble diameters at detachment agree with the experimental measurements; however, the model underestimates slightly the bubble formation times . The calculation of the various forces acting on the bubble in the vertical direction indicates that the real forces governing the bubble growth are the buoyancy force, the surface tension force, and near detachment the inertial force.