Partial oxidation (POX) and steam-CO2 mixed reforming of CH4 on MgO-supported noble metals were investigated at high space velocity (5.5 x 10(5) h(-1)). Temperature-programmed reaction (TPR) and isotope transient techniques were used to study the mechanism of POX and mixed reforming. TPR profiles of POX and mixed reforming showed similar ignition reaction behaviors, which implied that there are similar characteristics in their mechanisms. Steam reforming and CO2 reforming were found to start at the same time in mixed reforming. TPR and CH4-D-2 exchange experiments indicated that CH4 was activated at low temperature on Rh/MgO. POX showed much higher activity than mixed reforming although their C, H, and O atomic concentrations were the same at the beginning of each reaction. It is suggested that the lower rate of reaction in mixed reforming is due to the blocking of active sites for CH4 activation by CO2 and H2O. It seems that the coexistence of CO2 and H2O shows stronger inhibition than that of CO2 alone and H2O alone. Rh/MgO without previous reduction treatment also showed a high reactivity for POX but with a higher ignition temperature than a prereduced catalyst. With regard to the H-2/CO ratio, mixed reforming showed a changing ratio with increasing temperature, which suggested that the rate for CO2 reforming increases faster than that of steam reforming. An in situ isotope-labelled (CO2)-C-13 transient experiment for mixed reforming indicated that carbon formed from CO2 or CO decomposition was less active than (CHx)(ad) (x = 0, 1, 2, and 3) formed from CH4 decomposition. For POX, a small amount of steam had little effect on CO formation rate for an active catalyst, e.g., Rh/MgO or Ru/MgO, but decreased the rate for less active catalyst Pt/MgO. All the results indicated that steam reforming and CO2 reforming in mixed reforming start simultaneously and have the same type of reaction intermediate, adsorbed atomic oxygen. POX proceeds via both one-step and two-step mechanisms, the ratio for each mechanism being dependent on the concentration and kinetics of adsorbed atomic oxygen and gaseous atomic oxygen. Mechanisms for POX and mixed reforming are suggested acid the effect of oxygen-metal-bond strength on activity is discussed.