We describe an experiment that couples a high‐resolution Fourier transform spectrometer (FTS) to a supersonic jet of radicals. A 1‐mm‐i.d. cylindrical SiC nozzle is resistively heated to 1500 K in order to decompose organic precursors and generate expansions of jet‐cooled radicals. We have used this apparatus to pyrolyze alkyl nitrites to make alkoxy and nitric oxide radicals. The residence time of radicals in this hot nozzle is roughly 20 &mgr;s RONO→&Dgr;RO+NO. We use the FTS to detect the IR absorption of the product NO (&ngr;0=1876.1 cm−1) at resolutions as fine as 0.005 cm−1FWHM. We observe the product NO from the pyrolysis of CH3CH2ONO to be rotationally cooled to roughly 50 K. The IR spectra indicate that the optical path length is about 3/4 cm and that the nitrites pyrolyze to produce approximately 1014NO radicals cm−3some 9 mm downstream from the nozzle. Our spectrometer is capable of detecting an absorption signal of 0.1% over a bandwidth of 100 cm−1at 0.005 cm−1resolution. Depending on the infrared cross section of the radical, this implies that we are able to monitor diatomic radical densities of roughly 5×1012cm−3(quantum state)−1. ©1995 American Institute of Physics.