Mass transport in an air-breathing cathode of a proton exchange membrane (PEM) fuel cell was investigated numerically. The porous cathode in contact with a perforated current collector breathes fresh air through an array of orifices. The diffusions of reactant species in the porous cathodes are described using the Stefan–Maxwell equation. The electrochemical reaction on the surfaces of the porous cathode is modeled using the Butler–Volmer equation. Gas flow in the air-breathing porous cathodes is described using isotropic linear resistance model with constant porosity and permeability. The electron/ion transports in the catalyst/electrolyte are handled using charge conservation based on Ohm’s law. A finite-element scheme is adopted to solve these coupled equations. The effects of electrode porosity on the fluid flow, mass transport, and electrochemistry are examined. Detailed electrochemical/mass characteristics, such as flow velocities, species mass fraction, species flux, and current density distributions are presented. These details provide a solid basis for optimizing the geometry of a PEM fuel cell stack that is run in passive mode.