By means of advanced diagnostic tools - neutron imaging, pulsed gas method (helox or O-2 pulses during air operation) and electrochemical impedance spectroscopy (EIS) - three different flow fields are characterized in a differential (small-size cell operated at high stoichiometry) Polymer Electrolyte Fuel Cell (PEFC) to emulate the local behavior of a full-size cell. The channel-rib structures are designed so as to obtain 2D, 1D and 0D gradients of O-2 concentration across the gas diffusion layer (GDL). The important bulk-diffusion losses (evaluated with pulsed helox) of the "2D" cell compared to the "1D" cell indicate the effect of lateral oxygen diffusion in the GDL. Non-bulk diffusion losses (Knudsen and/or thin film diffusion, evaluated with pulsed O-2) also play a non-negligible role on the overall diffusive losses. In particular, their increase in dry conditions for the "1D" and the "0D" cells might be a consequence of drying effects in the electrode. EIS spectra can be interpreted consistently with the results of the pulsed gas method: the increase of arc diameter can be correlated with the overall mass transport losses including both bulk and non-bulk diffusion losses. (C) 2013 The Electrochemical Society. All rights reserved.