In this study the mass transport resistance in the cathode catalyst layer (CCL) of a polymer electrolyte fuel cell (PEFC) is estimated using electrochemical impedance spectroscopy (EIS) measurements. Experimental impedance measurements were carried out in a 6 cm(2) PEFC operated with two different relative humidity (RH) values in the cathode and different partial pressures of oxygen in He/O-2 and N-2/O-2 gas mixtures. A mathematical model predicting the CCL impedance response, derived in the authors' previous study, is applied to EIS measurements to calculate the CCL mass transport resistance. The experimental results show the presence of an overlapped second semicircle at low frequencies which is attributed to an increase in the time constant to diffuse oxygen through the CCL when the PEFC is operated at low oxygen partial pressures, p(O-2) <= 20%, in He/O-2 or N-2/O-2 gas mixtures. The results also show that oxygen diluted with nitrogen can reduce the steady state oxygen concentration in the CCL-gas diffusion layer (GDL) interface and can increase CCL mass transport resistance. It is possible, as such, to harness capabilities from both modelling and real-world EIS data in a complementary manner. (C) 2013 Elsevier B.V. All rights reserved.