The H-2-O-2 proton-exchange membrane (PEM) fuel cell, among numerous other potential applications now slated to provide the motive power for the next generation of highly efficient and largely pollution-free automobiles, is an incomparable membrane reactor, comprising an exquisitely designed membrane-electrode-assembly (MEA), a five-layer composite of two gas-diffusion layers, two supported-catalyst layers, and a PEM. The device allows catalytic reaction and separation of hydrogen and oxygen as well as protons and electrons. This paper describes the structure and performance of the PEM fuel cell considered as a membrane reactor and develops an analytical transport-reaction model that, despite some assumptions, captures the essential features of the device very well. The key assumptions are that transport resistance as well as ohmic drop are negligible in the catalyst layer. While the latter is defensible, the former causes deviations at high current densities. Nonetheless, the model predicts the fuel cell performance well with parameter values reported in the literature.