A composite storage tank comprises a composite wall enclosing a gas storage volume and defining a cylindrical portion of the tank. The composite wall incorporates first and second sets of metallic fibres each of which is susceptible to embrittiement by hydrogen and has ends extending through the exterior surface of the composite wall. By measuring the electrical resistances of the metallic fibres, a measure of the amount of hydrogen that has leaked through the composite wall over a period of time, and the present physical condition of the tank, may be determined. The approximate axial and azimuthal coordinates of a particular leakage point may also be determined.