The effect of the operating conditions, e.g., load, temperature, relative humidity (RH), and the MEA's aging condition oil the pH of the water drained out from the cathode and anode sides of a H-2/air PEM fuel cell was studied. Also the effect of the pollutants' existence in natural air on the measured pH and the performance of the fuel cell was investigated. pH values as low as 1 were measured for the water drained out from the cathode side under a low temperature-low RH condition. Increasing the load, temperature or RH value resulted in an increase of the measured pH except for the low temperature-low RH condition where increasing the load resulted in a decrease in the measured pH. On the other hand, the pH value of the water drained out from the anode side was around 4 under the same low temperature-low RH condition. Aging of the MEA at 90 degrees C and RH of 100% for at least 30 h resulted in low measured pH values for the water drained out from the cathode side. The polarization behaviors of the cathode under these different conditions were measured and correlated to the pH change and the performance of the MEA. Measuring the pH using a flow pH meter for the water droplets drained out from the cathode side can be used as an alarm for the onset of the chemical degradation of the Nafion membrane. (c) 2009 Elsevier B.V. All rights reserved.