The degradation of the polymer electrolyte is one of the dominant factors, which limits the lifetime of the polymer electrolyte fuel cells (PEFCs). Therefore, the understanding and mitigation of all contributing degradation phenomena is fundamental in order to establish the PEFC as competitive product. Membrane degradation is often quantified by analysing the fluoride emission rate by determining the fluoride content in the effluent water. However, when oligomers (i.e. larger polymer fragments) are released, the ion conductivity of the ionomer declines without extensive fluoride release, as the carbon backbone fluorine bonds are still intact. Thus, it is necessary to conduct effluent water treatment in order to determine the total fluorine emission rate (tFER) instead of the fluoride emission rate (FER) only [1]. In the presented work, an accelerated stress test (AST) was conducted in a single cell with a segmented S++ current scan shunt device with a sensor plate on the cathode for spatial current and temperature measurements in 10x10 and 5x5 segments, respectively. A membrane electrode assembly was characterised in situ [2]. Throughout 100 hours of stress testing, the effluent water was collected and used to determine the FER, using a fluoride selective electrode (Figure 1) [3]. Furthermore, effluent water samples were treated with KOH following different protocols. The alkaline treatment decomposes residual ionomer fragments for the detection of the total fluorine emission rate. The tFER allows more profound assumptions regarding the extent of the membrane degradation and further, gives insight into the underlying degradation mechanism. In preliminary results it is shown that by conducting alkaline effluent water treatment at a temperature of 500 °C the amount of detected fluoride was increased significantly (Figure 2). Thus it can be concluded that not all emitted fluorine is detectable without previous treatment, indicating that the electrolyte membrane decomposes into oligomeric fragments. Acknowledgement The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement n° 621216. [1] Aarhaug TA, Svensson AM. Degradation Rates of PEM Fuel Cells Running at Open Circuit Voltage. ECS Trans., vol. 3, ECS; 2006, p. 775–80. doi:10.1149/1.2356197. [2] Bodner M, Cermenek B, Rami M, Hacker V. The Effect of Platinum Electrocatalyst on Membrane Degradation in Polymer Electrolyte Fuel Cells. Membranes (Basel) 2015;5:888–902. doi:10.3390/membranes5040888. [3] Bodner M, Hochenauer C, Hacker V. Effect of pinhole location on degradation in polymer electrolyte fuel cells. J Power Sources 2015;295:336–48. doi:10.1016/j.jpowsour.2015.07.021. Figure 1