The autothermal catalytic partial oxidation (CPO) of methane was performed at short contact times (similar to 8 ms) over three Rh-based catalysts: 5 wt% Rh/alpha-Al2O3, 5 wt% Rh/2 wt% gamma-Al2O3/alpha-Al2O3, and 5 wt% Rh/2 wt% Ce/2 wt% gamma-Al2O3/alpha-Al2O3. The effects of H2O addition (10%, 20%, and 40% of the total feed) and CO2 addition (20% and 35%) were studied over a wide range of inlet C/O ratios (0.75-1.2) by means of the capillary sampling technique. Over Rh/alpha-Al2O3 samples, spatially resolved concentration profiles revealed that the rate of CH4 reforming is independent of the concentration of H2O and CO2. Differences in the product distribution followed the behavior expected from water gas shift (WGS) chemistry: in H2O-rich tests, the production of H-2 and CO2 increased at the expense of H2O and CO2 while the opposite was observed in CO2-rich tests. CPO experiments with a simultaneous feed of H2O and CO2 (40% H2O, 20% CO2, 20% CH4, C/O = 1) provided direct evidence that H2O is the preferential co-reactant of CH4 in reforming and that CO2 reforming is absent. Addition of a gamma-Al2O3 washcoat to the catalyst significantly enhanced the rate of steam reforming while revealing the limits of Rh WGS activity. Products were equilibrated for nearly all cases examined over the Ce-promoted catalyst, indicating that WGS kinetics, either forward or reverse, have an important role in the CPO mechanism, particularly with H2O and CO2 co-feed. The experimental results clearly illustrate the flexible nature of CPO process. They show that the H-2/CO can be varied within a wide range of values in a one-step process, and syngas production is sustainable under highly diluted conditions. (C) 2009 Elsevier Inc. All rights reserved.