Thermally persistent triplet sulfonyl nitrene, FSO2N, was produced in the gas phase in high yields (up to 66%) by flash vacuum pyrolysis of FSO2N3. Surprisingly, no rearrangement of FSO2N was observed, but the long-sought radical FSO2 (22%) and traces of SO2 (3%) were identified by IR (N-15, O-18, S-34) spectroscopy. The photoinduced Curtius rearrangement of the nitrene to FNSO2 was observed in solid noble gas matrices, and reactions of the nitrene with O-2, NO, and CO were studied.