The normal operation of polymer electrolyte membrane fuel cells (PEMFCs) can induce significant temperature, humidity, pressure or concentration gradients across the cell's active area. The overall performance and stability can be affected negatively by these gradients, and they may contribute to material degradation, and ultimately, to cell failure. We report on diagnostic techniques and methodologies to characterize PEMFC material properties and the inhomogeneities across the active area of a working cell. Specifically, we describe in-situ techniques for the spatially-resolved characterization of PEMFC performance. Our testing hardware features sixteen fully isolated segments over a 49 cm2 active area, reference electrode capabilities, and individual segment control. The techniques are applied to the in-situ characterization of effective platinum surface area (EPSA) degradation under 1×105 accelerated stress testing (AST) cycles. © The Electrochemical Society.