This report describes work performed to improve porous electrode and mass transfer models used in calculating the performance of molten carbonate fuel cells. The report is comprised of three parts. Parts I and II contain the mathematical model development. Porous electrode modeling is addressed in Part I. Gas diffusion within electrode pores is treated. A method is developed for computing electrode performance that includes the effects of limited gas phase diffusion and finite electronic conductivity of the electrode. A heterogeneous film model is proposed as a representation of some anodes currently under development. In Part II, the formation of vertical concentration gradients in the gas distribution channels is considered. An improved mass transfer scheme is developed in order to include the effect of this pehnomenon on cell performance. Part III contains listings of the computer models developed and/or modified during this research.