Micron-and millimeter-scale sensors were employed to acquire first-of-their-kind experimental measurements of the spatial and temporal distributions of temperature, oxygen partial pressure, and relative humidity in the mass transport layer immediately above the planar, horizontal cathode of polymer electrolyte membrane fuel cell (PEMFC) driven by natural convection. The sensors provide approximately 1 mm (or better) spatial resolution and 1 s temporal resolution. Substantial changes in temperature and reaction species concentrations were observed with increasing current density during a current-voltage (I-V) scan. A linear decrease in oxygen partial pressure and a linear increase in water vapor partial pressure were observed with increasing current density, consistent with a flux balance analysis. Spatially resolved profiles normal to the surface indicate that thermal and reaction species gradients extend up to 6 mm above the horizontal cathode surface. Complementary horizontal profiles (parallel to the surface) reveal that the cell's cathode rib structure visibly influences oxygen distribution. Most significantly, these data show that thermal and species concentration effects are not confined to the gas diffusion layer (GDL), but extend well beyond the cathode surface, into the surrounding space. The measurements were used to estimate diffusion and/or convection mass transfer coefficients above the cathode surface. Transient data reveal substantial differences in the time constants associated with oxygen, water, and heat transport. The insights provided by this study should prove useful to inform and validate future physical models of air-breathing fuel cell systems. (C) 2007 The Electrochemical Society.