This Small Business Innovation Research (SBIR) Phase I project will develop compact Water Gas Shift Reactors (WGSR) with rapid startup and load following through the use of a novel catalyst substrate design consisting of multiple Ultra Short Channel Length (USCL), high cell density metal monoliths in series. These USCL monoliths have very high heat and mass transfer coefficients due to the absence of fully developed boundary layers; this increases bulk mass transfer on the order of 20 fold over conventional honeycomb monolith supports. The high cell density, up to 2500 cells per square inch, results in a considerably higher Geometric Surface area (GSA) per unit volume compared to honeycomb monoliths. The improved transport properties and increased GSA translates into much smaller reactor size and weight compared to pellet bed or conventional honeycomb substrates and more efficient catalyst utilization under mass transfer controlled operation, which can lead to significant cost reductions, especially when using precious metal catalyst. The very low thermal mass of the individual USCL catalyst substrate elements combined with the high heat transfer coefficient gives improved transient response and fast startup.<br/><br/>The proposed WGSR catalyst technology is primarily intended as part of an integrated fuel processor system to produce hydrogen for Proton exchange Membrane fuel cells in automotive applications. The proposed technology provides a very high potential benefit to cost opportunity, offering significant improvements in the WGSR component of volume, weight and potentially cost, as well as provides spin-off applications to other catalytic reactors (including other fuel processor components).