Autothermal CH4 catalytic partial oxidation (CPO) tests were performed in an adiabatic lab-scale reformer over 2 wt% Rh/alpha-Al2O3 catalysts supported on 600 cpsi cordierite honeycombs. The effect of adding either CO2 or N-2 to CH4/air mixtures was investigated at constant O-2/CH4 ratio (0.56) and total flow rate (10 Nl/min). At increasing dilution the conversion of CH4 and the gas temperatures decreased. Also, at equal CH4 concentration, the addition of CO2 decreased the temperatures and the conversion of CH4 more importantly than N-2. Thus, in line with previous studies from this and other laboratories, no evidence of a direct dry reforming route was found; the observed thermal behavior appeared largely controlled by the heat capacity of the feed streams. However an important chemical effect of CO2 addition, not previously appreciated in the literature, was shown by the evolution of the outlet H-2/CO ratio; at increasing N-2 dilution, the H-2/CO ratio slightly increased from about 2 to 2.5, but at increasing CO2 dilution, the H-2/CO ratio decreased from about 1.7 to 0.7. These trends are in line with the thermodynamics of the reverse water gas shift (RWGS) reaction; such a reaction was thus extremely fast in all the experiments with CO2, even at the highest values of dilution, while CH4 conversion did not reach the equilibrium. A C-1 microkinetic model was used to analyze the results. A close match between calculated and measured temperatures, conversions and syngas composition was obtained under all conditions. Notably, the present microkinetic scheme, which incorporates steps for CO2 adsorption and reactivity, was able to account for the observed net consumption of CO2 and for the lowering of the H-2/CO ratio in the experiments with CO2 co-feed. Surface coverages were analyzed for the various investigated conditions and the effects on the kinetics of methane steam reforming were evaluated in detail; the factors which control the onset of a more kinetically controlled regime at high degrees of dilution were highlighted. (C) 2010 Elsevier B.V. All rights reserved.