The oxidative dehydrogenation of C2H6 and C3H8 have been studied at extremely short contact times in the order of 10(-5) s in quartz reactors using Pt or Pt/Rh gauze catalysts. The reactions were studied at 1 bar and gauze temperatures of 600-1050 degrees C (for C2H6) and 695-935 degrees C (for C3H8). The alkane/O-2 ratio of the feed was typically 1.7, Ar was normally used as diluent (30 - 79 vol %). However, some runs have also been performed with steam as the diluent giving similar product selectivities. At low and moderate conversions higher C2H4 selectivities were always obtained in an empty reactor than in presence of the gauze catalyst. The product composition approaches the same values as in the presence of a catalyst at more severe conditions giving conversion levels above approximately 55%. The role of the gauze is to facilitate the ignition of the reaction mixture and to provide extremely rapid in situ heating of the reactants by combustion. However, the oxidative dehydrogenation seems to proceed mainly in the gas phase. Synthesis gas formation is important at high conversion, especially in case of C2H6. A H-2/CO ratio close to 1.5 (from C2H6) and close to 1 (from C3H8) was obtained at high conversions also favoring the formation of C2H4 . C2H2 and benzene are favored by high reaction temperatures.