We report an in situ measurement of the oxygen diffusion coefficient D-ox in the cathode catalyst layer (CCL) of a direct methanol fuel cell (DMFC). The method is based on measuring two DMFC impedance spectra for the same cell current, but different oxygen concentrations in the cathode channel. The spectra are fitted using a transmission line model to extract the CCL resistivity. The difference of two CCL resistivities for different oxygen concentrations determines D-ox. We found D-ox = 0.838 x 10(-4). exp (-E-a/R(1/T-1/303)) cm(2) s(-1) in the temperature range from 30 degrees C to 90 degrees C, with E-a similar or equal to 20.1 kJ mol(-1). The activation energy is not far from that of the oxygen diffusion in Nafion; the value of this parameter is discussed. (C) 2014 Elsevier Ltd. All rights reserved.