Experiments of H-2 and CO fuel rich combustion, performed in an isothermal annular reactor over a Rh-catalyst, are herein analyzed by means of a 2D isothermal model. The model incorporates previously derived molecular surface kinetics of H2 and CO combustions as well as established detailed kinetic schemes for H2 and CO gas-phase combustions, developed by Ranzi and co-workers, which allowed to evaluate the relative role of heterogeneous and homogeneous reactions. In line with the experimental evidence, homogeneous ignition of the H-2/O-2 mixture was predicted to occur above 650 degrees C (though it was retarded by the presence of the surface reaction), while this did not occur for the CO/O-2 mixture (only heterogeneously consumed). For both reaction systems, intermediate temperature windows were identified wherein the observed conversions exceeded the diffusion limit and could not be explained by the combined heterogeneous/homogeneous model. Radical seeding simulations showed that a low-temperature promotion of the gas-phase conversion of H2 can be explained by a proper temperaturedependent enrichment of radical species; however, the effect seems hardly consistent with the desorption of adsorbed intermediates, thus the indications are not conclusive on the existence of an interaction between surface and gas-phase chemistries. (c) 2007 Elsevier Ltd. All rights reserved.