Polymer electrolyte membrane water electrolysers today mainly still use classic extrusion cast PFSA membranes of significant thickness such as Nafion® N117 or N115. In PEM fuel cells, much thinner solution cast membranes are used today in order to reduce ohmic resistance. In spite of the fact that reduced ohmic resistance would also benefit the goal to increase current density of operation for PEMEL so far it is not done for the sake of limiting hydrogen and oxygen cross-over. The necessity of this arises from the fact that hydrogen in oxygen concentrations exceeding certain limits can cause severe safety risk for the operation of PEMEL systems. An alternative approach to mitigate the issue is the introduction of recombination layers1, 2 which scavenge hydrogen and oxygen within the membrane and combine them back into water. In order to appropriately design such layer an online measurement of the hydrogen cross-over is required.In this contribution we will report on the use of online mass spectrometry to measure and quantify the hydrogen cross-over. The used set-up consists of a balticFuelCells cell holder with 4 cm2 cell with titanium flow fields as well as two pumps to supply water to anode and cathode compartment. Downstream of the cell on the anode side a water separator removes the remaining water from the product gas line. The capillary of a mass spectrometer with differential pump system is inserted into the exhaust gas line. Hydrogen concentration was measured by monitoring the m/z = 2 signal. For the calibration of the cell only a PTL was mounted into the cell holder. During the calibration measurement water was flown through the anode compartment while the anode side was fed with a 5% H2 in N2 mixture. The observed m/z = 2 signal was set for 5 vol% of hydrogen.For the tests, home-made CCMs were used with a Pt/C (JMFC) cathode layer and an IrO2 (Alfa Aesar) anode layer. A Freudenberg GDL was used as cathode PTL and a sintered Ti element as anode PTL. Tests were performed for CCM using Nafion® N115 or Nafion® XL electrolyte membrane. For some of the experiments a Nafion® overlayer was applied to the Nafion® XL to further determine the effect of Nafion® thickness. Also, a first test with a Pt black layer as recombination layer insert between the Nafion® XL membrane and the Nafion® overlayer was performed.Results reveal that the hydrogen cross-over for the Nafion® XL membrane is significantly higher than for the Nafion® N115 membrane as has to be expected. Already a rather thin Nafion® over cast to the Nafion® XL membrane can reduce the cross over to the Nafion® XL level without the need to achieve the full thickness of the Nafion® N115 membrane. The addition of the Pt layer is very effective to reduce H2 cross-over and will be further investigated. Of further importance is the strong spike of H2 cross over during current increase slopes which can be observed with this technique. These spikes bear the risk of the transient existence of flammable mixture. The observation is only possible due to the high ti...