A model of the air electrode used in fuel cells was developed which accounts for the diffusion of oxygen in the gas-filled pores as well as diffusion into the liquid-filled pores, and electrochemical reaction. The model was applied to the air electrode in the phosphoric acid fuel cell (PAFC) to simulate the cathode performance under a variety of conditions. Two parameters AB and AI were introduced into the model. They reflect, respectively, the effects of the interfacial surface area between the gas and liquid phases and those of the interfacial surface area between the liquid and solid phases, on the air electrode performance. AB and AI can be determined with the aid of the operating conditions and potential-current density curves of the air electrode. The main interface parameters of the air electrode can be predicted following the determination of AB and AI. The reaction rate throughout the catalyst layer was calculated by means of the model.