A 1-D down-the-channel two-phase pressure drop modeling scheme has been developed for predicting pressure drop in PEM fuel cell reactant channels. The model is based on local conditions taking into account continuous reactant consumption, water production, membrane transport and change in local saturation conditions along the flow direction. The channel is divided into discrete elements in the flow direction, and a step-wise marching scheme is employed to determine the local air and water flow rates, and local saturation condition. The model is validated using ex situ and in situ experimental data obtained over a range of operating conditions, typical of automotive fuel cell operation. The current density (0.1-1.0 A/cm 2 ), inlet humidity (0, 50 and 95% RH) and stoichiometry (1.5-8) are systematically varied to study the fuel cell pressure drop response for a total of 198 operating conditions. The modified form of the correlation proposed by English and Kandlikar has been shown to provide the most accurate pressure drop predictions. The two-phase pressure drop on the cathode side can be predicted with a mean deviation of 11.6% using the proposed scheme.Reactant gases flowing in the gas channels of Proton Exchange Membrane (PEM) fuel cells interact with the water being generated as part of the reaction and condensation from the stream, resulting in two-phase flow in the channels. Overall size restrictions on the fuel cells required to meet the US -Department of Energy (US -DOE) targets for volumetric power density (640 W/L) result in the use of minichannels, with hydraulic diameters on the order of 500 μm 1 . The two-phase flow configuration in the reactant channels of PEM fuel cells is unique in several ways. The channels are bound on one side by the gas diffusion layer (GDL), which is a porous medium with very high hydrophobicity (contact angles in the range of 110 • -140 • ). Whereas all the two-phase flow studies have been conducted with uniform gas and liquid flows through the length of the channel, in PEM reactant channels, water is introduced continuously along the length of the channel and the reactant gas is consumed along the length. This results in a continuous change in both air and water flow rates on the cathode side. The local quality (gas flow fraction) thus changes in response to changes in the flow rates. In addition, the water transport across the membrane from anode to cathode side and local evaporation or condensation of water causes changes in the water present in liquid form due to varying saturation along the flow length.Understanding the two-phase flow is important for improvement of reactant distribution (flow patterns) and pressure drop prediction. The channel pressure drop directly influences the pumping power required to supply the reactants. Any additional parasitic power (such as an increase in pumping power) results in the reduction of system efficiency. Bosco and Fronk 2 showed a direct link between the pressure drop and the overall cell performance through the cost and sizing of f...