This study describes a method using mass spectrometry (MS) to quantify the gas barrier properties of different classes of ionomer membranes and the base film used in radiation grafted membranes for polymer electrolyte fuel cells. The effects of relative humidity (RH), temperature and graft level are investigated for different types of radiation grafted membrane with dissimilar chemical compositions. The results show that the radiation grafted membranes and Nafioe (R) 212 share the same trend of increasing gas permeability with RH, while the ETEE base film does not show RH dependency. With addition of nitrile-containing comonomer units in the grafted chains, the permeability of reactant gases in the membranes can be remarkably reduced. On the basis of experimental results, a physical model is proposed to describe the effect of water contents on the gas transport in the radiation grafted membranes and to explain the effect of graft level on gas barrier properties and the difference of gas barrier properties at high and low hydration levels between Nafioe (R) membranes and radiation grafted membranes. Using helium as a tracer, the deterioration of the gas barrier properties of the radiation grafted membrane during an accelerated stress test at open circuit voltage (OCV) is recorded in situ. (C) 2014 Elsevier B.V. All rights reserved.