A method for the high‐temperature generation of reactive species in supersonic molecular beams has been developed. Pyrolysis temperatures (25–1800 °C) can be maintained for longer than 100 h for extended experiments. Short contact time (≊10 &mgr;s) of the seeded (<1 Torr partial pressure in 2 atm inert carrier gas) precursor molecules suppresses recombination of the product radicals. Number densities for radicals are estimated to be ≊1014cm−3at the sonic orifice with radical fluxes of ≊1016s−1.