The reaction pathways of ethane and propane partial oxidation to synthesis gas were investigated over a Pt/gamma-Al2O3 and a Rh/alpha-Al2O3 catalyst. An annular reactor was used for this purpose at high space velocities and at temperatures below 700degreesC in order to avoid homogeneous reactions. Under fuel-rich conditions, the Pt-based catalyst produced CO and H-2 at high temperatures (>550 degreesC), while CO2 and H2O were the only reaction products at lower temperatures. The formation of CO and H-2 was consistent with direct oxidation reactions, since contact time had no effect on the product distribution, and secondary reactions (steam and dry reforming) showed negligible activity. The Rh/Al2O3 catalyst was also active and selective in the partial oxidation of light hydrocarbons, but in this case the production of hydrogen and CO was strongly dependent on contact time, and steam reforming was important even at short contact times. It was concluded that. over rhodium, both direct and indirect routes were probably involved in the formation of CO and H-2. The main difference between the two noble metals thus seemed to be that Pt mainly produced CO and H-2 by means of O-2 (direct routes), while over Rh the light paraffins were converted to CO and H-2 by means of O-2 and H2O (direct + indirect routes). This could explain the remarkably different behavior of the two systems when tested in high temperature autothermal reactors (T > 700 degreesC). Under adiabatic conditions, the partial oxidation of light paraffins led to large amounts of gas-phase olefinic products over Pt, whereas high selectivities to synthesis gas were found over Rh. The mechanistic results suggest that this different behavior could be due to the varying capability of Pt and Rh surface reactions to compete with homogeneous reactions. (C) 2003 Elsevier B.V. All rights reserved.