We report a method for in situ determination of the oxygen diffusion coefficient D in the cathode catalyst layer (CCL) of a PEM fuel cell. The method is based on our recent model equation for the CCL polarization potential. Two low-current impedance spectra of the CCL should be measured for the same cell current and different oxygen concentrations. We show that D is given by D = (k - 1)bl(t)/(k12F delta R(ccl)c), where b is the Tafel slope, l(t) is the CCL thickness, delta R-ccl is the difference of the low-frequency CCL resistivities, c is the smallest oxygen concentration used in experiments, and k > 1 is the ratio of two concentrations. (C) 2014 Elsevier B.V. All rights reserved.