The long term behaviour of two 100 kW proton exchange membrane (PEM) water electrolyser plants is analysed. The systems had to be shut down due to problems with excessive levels of hydrogen in the oxygen product stream. The time to breakdown was different by a factor of nearly 10 from plant to plant. Post mortem analysis of the cell stacks revealed that the Nafion(R) 117 membrane is the weakest part in a PEM electrolyser regarding long term performance. Substantial thinning of the membranes in the stacks was detected. The degradation process was found to depend on the position within an individual cell, as well as of the position of the cell in the electrolyser stack. The dissolution process proceeds from the interface between the cathode and the membrane, is not specific with respect to the ion exchange groups, and is most likely triggered and/or enhanced by local stress on the membrane.