Abstract: The invention relates to a method for operating an electrolysis plant having an electrolyser for generating hydrogen (H2) and oxygen (O2) as product gases, with water being supplied as starting material and being split at a proton-permeable membrane into hydrogen (H2) and oxygen (O2), a product gas stream being formed in a phase mixture comprising water (H2O) and a relevant product gas, and a product gas stream being supplied to a gas separator arranged downstream of the electrolyser, characterized in that the fluoride release of the membrane is determined on the basis of the operating time, the temporal progression of the fluoride concentration being ascertained, with a measure for the operation-induced degradation of the proton-permeable membrane being ascertained as the result of a release of fluoride. The invention furthermore relates to a corresponding electrolysis plant and to a measuring device for carrying out the method.