The production of fuel-cell-grade hydrogen, containing very low levels of CO, is critical for the global transition to a hydrogen society. In spite of ongoing efforts to make fuel cells more CO-tolerant, [1] the use of hydrogen fuel-cells in portable applications (e.g., automobiles, battery replacement devices) is particularly challenging, as the production of hydrogen by steam-reforming of hydrocarbons [2] requires a complex combination of multiple processes [3][4][5][6][7] to achieve the required low CO levels (e.g., 100-500 ppm).[1] An important step toward a simple process for the production of hydrogen containing low levels of CO is made possible by the discovery [8] that hydrogen can be produced by catalytic reforming of biomass-derived oxygenated hydrocarbons in liquid water at temperatures near 500 K. This process has the advantage that the reforming of oxygenated hydrocarbons and the water-gas shift (WGS) reaction are both thermodynamically favorable at the same low temperatures, thus making it possible to conduct both reactions in one reactor. These low levels of CO, combined with the fact that our low-temperature process uses biomass-derived renewable feedstocks, makes aqueous-phase reforming of oxygenates an attractive process for applications in portable devices.Herein, we show results from studies of the aqueousphase reforming of ethylene glycol (EG) to produce H 2 , with specific emphasis on the levels of CO produced as a function of the process conditions. We show the importance of the reaction operating conditions for achieving low levels of CO in the effluent gas from our single-reactor reforming process, and we elucidate the factors that control the ultimate levels of CO that can be attained.Aqueous-phase reforming of oxygenates to produce H 2 takes place on metal catalysts [9] through CÀC cleavage to make CO and H 2 , [Eq. (1)].(Undesired alkanes are formed through C À O cleavage.) Adsorbed CO undergoes WGS [Eq. (2)], which increases theamount of H 2 produced and removes CO from the catalyst surface.[10]The lowest partial pressure of CO that can be achieved depends on the thermodynamics of the WGS reaction and the operating conditions, [Eq. (3)].K WGS is the equilibrium constant for the vapor-phase WGS and P j are partial pressures. Since H 2 , CO 2 , and small amounts of alkanes (primarily CH 4 ) are produced by aqueous-phase reforming, [8] gas bubbles are formed within the liquid-phase flow reactor. The pressure in these bubbles can be approximated to be equal to the system pressure, [Eq. (4)].The partial pressures of the reaction products and water vapor are dictated by the feed concentrations, system pressure and temperature, as outlined below. For dilute product concentrations and system pressures above the saturation pressure of water, the bubbles contain water vapor at a pressure equal to its saturation pressure at the reactor temperature, and the remaining pressure is the sum of the partial pressures of the product gases. The extent of vaporization, y, is defined as the percent of wate...