The water-gas-shift (WGS) reaction is an important industrial reaction used to remove CO from synthesis gas during H₂ production.  The reaction is gaining importance as a consequence of increased interest in producing H₂ for fuel cells.  The widely used low temperature WGS catalyst formulation, Cu-Zn-Al, has low intrinsic activity resulting in relatively large reactor volumes.  Recently we discovered that the introduction of Group VIII metals onto Mo₂C significantly improved the WGS activity.  In fact, some of the carbide-based formulations were more than an order of magnitude more active than a commercial Cu-Zn-Al catalyst.  In this paper we discuss the WGS mechanisms for Mo₂C and Pt/Mo₂C catalysts.  The data was fitted to the power law, Langmuir-Hinshelwood-Hougen-Watson (LHHW), and redox models.  The results suggested that mechanisms for the carbide-supported materials were different from that for the Cu-Zn-Al catalyst.  The WGS rate for Mo₂C was nearly first order in CO concentration and almost zero order with respect to H₂O.  These orders were essentially reversed with the introduction of Pt.  The Cu-Zn-Al catalyst was nearly first order in both CO and H₂O.  The zero order dependency on CO for the Pt/Mo₂C makes it superior to Cu-Zn-Al at low CO concentrations.  The reaction over Mo₂C proceeds through a mechanism in which CO and H₂O compete for adsorption sites. Results for the Pt/Mo₂C catalyst suggested a dual site mechanism involving the non-competitive adsorption of CO and H₂O.  The reaction rate for Pt/Mo₂C was best described using a LHHW model with surface reaction being the controlling step.  The beneficial role of Pt was also evident from the CO and H₂O chemisorption data.  Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and in-situ pulse reaction studies were also used to provide additional mechanistic understanding.  In addition, the models were used to estimate sizes for the WGS reactors.  Reactors incorporating the Pt/Mo₂C catalysts are projected to be an order of magnitude smaller than that for a Cu-Zn-Al catalyst.  These and other results will be discussed.