The reactivity of carbon species formed from methane decomposition over supported noble metal catalysts was examined. In the Ru-loaded case, the rate of formation of CHx species in the decomposition of CH4 and the amount of CHx species over Ru are independent of the type of support. However, the carbon species on Ru/La2O3 seems to be more uniform and reactive than that on Ru/Al2O3. After the decomposition of CH4 over Ru-loaded catalysts was carried out at 700 degrees C, a temperature-programmed reaction with D-2 was carried out. CD4 was the major product on all the supports, and only slight amounts of CH3D, CH2D2, and CHD3 were detected over La2O3, ZrO2, and Y2O3 supports. The reactivity of CHx species on Ru strongly depends on the supports. The reactivity of CHx species on Ru and Rh loaded on La2O3 can be divided into two categories, These categories are not based on the differences in the number of x in CHx species. On Ru, x in the CHx species is equal to nearly 0, with a very small portion having x = 2. On the other hand, x is almost exclusively 0 on Rh. It seems that Rh leads to a higher rate of decomposition for methane than does Ru.