Cryogenic gas chromatography (GC) combined with isotope labeling was applied to discriminative determination of hydrogen gas formed through consecutive CO2 reforming of methane and water-gas shift reaction. First, hydrogen gas was obtained by using a tandem reaction system, where reforming and shift reaction occurred consecutively. Here, D2O instead of H2O was used for shift reaction to label hydrogen gas generated there. Then, the resulting hydrogen isotopes were analyzed by a cryogenic GC system, in which an oven temperature was maintained at -196 degrees C by immersing a separation column into liquid nitrogen. On the chromatograms, the peaks of H-2 and D-2 generated from reforming and shift reaction, respectively, were clearly observed as well-separated peaks. Based on their peak intensities, hydrogen gas formed through the tandem reaction system was precisely determined with differentiating its sources. (C) 2006 Elsevier Ltd. All rights reserved.