The partial oxidation of CH4 over rhodium coated monoliths and C2H6 over platinum coated monoliths was examined in the fuel rich regime at : space velocities up to 4 x 10(6) h(-1) (similar to 200 mu s), a variation by a factor of 50. For CH4, the conversion drops from 90% at 5 x 10(4) h(-1) to 20% at 1 x 10(6) h(-1), while for C2H6, the conversion and the selectivity to C2H4 remain nearly constant up to 4 x 10(6) h(-1). For CH4, the O-2 conversion drops below 100% at 5 x 10(5) h(-1) but remains complete in the oxidation of C2H6. AS the space velocity increases in CH4 oxidation, the temperature of the leading edge of the monolith decreases from 900 degrees C at 1 x 10(5) h(-1) to 60 degrees C at 1 x 10(6) h(-1). The cool region of the monolith promotes the formation of total oxidation and also the production of CH3 radicals which lead to the formation of C-2 coupling products in the gas phase. A maximum in coupling selectivity of 10% is observed at 5.5 x 10(5) h(-1) and 270 degrees C preheat.