Our previous models and a catalyst-layer model are combined to study the effects of microporous layers in polymer electrolyte fuel cells. The combined sandwich model is used to fit data both with and without a microporous layer with saturated feed conditions. In terms of water management, the simulations clearly show that the effect of a microporous layer is to keep water out of the cathode gas diffusion layer and move it through the anode. Additional effects of microporous layers include better ohmic behavior and perhaps better catalyst utilization, among other things. Optimizations for different structural parameters are investigated, as are the effects of microporous layers under different operating conditions. The discussion is geared toward how microporous layers increase performance and their effect on fuel cell water management. (c) 2005 The Electrochemical Society. All rights reserved.