A long-term contamination (for 1000 hrs) test is conducted under constant current operation with 5 ppm acetonitrile in the air stream, and the results are compared to those for a controlled blank single-cell test. The long-term effects of the contaminant on cell performance and the structure of the membrane electrodes assembly are identified by a series of analyses, including electrochemical impedance spectroscopy, cyclic voltammetry, ion chromatography, transmission electron microscopy and scanning electron microscopy. During the long-term test, the acetonitrile significantly decreases the cell performance for a 0.1-mg Pt cm(-2) catalyst coated membrane. The acetonitrile contamination accelerates catalyst particle growth; however, interestingly, it inhibited membrane degradation. This result may provide a possible approach for improving fuel cell durability. (C) The Author(s) 2018. Published by ECS.