Catalysts monoliths with circular and square ducts are theoretically analyzed in detail as reactor configurations for the adiabatic CH4 partial oxidation on Rh for short-contact-times hydrogen production. By the means of CFD coupled with a detailed microkinetic description of the surface reactivity, it was found that the different transport properties of the investigated configurations primarily affect the thermal behavior of the reactor. O-2 consumption is fully external mass transfer limited, and thus, local variations in mass transport properties are responsible of the differences in surface temperature.