Catalytic partial oxidation (CPDX) is a promising technology for reforming of liquid hydrocarbon fuels to hydrogen or synthesis gas for use in fuel cells. The addition of a certain amount of the tail gas of the fuel cell stack to the reformer inlet feed can increase overall efficiency and lead to higher H-2 and CO selectivities and reduce coke formation. The effect of carbon dioxide or steam addition (1, 5, 10, 20, and 30 vol% of the total flow) on the performance of a CPDX reformer operated with isooctane as fuel surrogate is systematically studied over a wide range of C/O feed ratios (0.72-1.79) using a Rh/alumina honeycomb catalyst. The specific impact of the coreactants H2O and CO2 on reformer behavior can be interpreted by the water gas shift (WGS) chemistry. Production of H-2 and CO2 increases with H2O addition at the expense of CO and H2O. Opposite trends are observed in case of CO2 addition. Tail gas recyding reduces formation of soot precursors up to 50% compared to the corresponding fuel feed without coreactants. However, tail-gas recycling shifts the formation of soot precursors toward lower C/O ratios.