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The haloformonitrile oxide BrCNO has been generated for the first time in the gas phase from the dibromoformaldoxime precursor by pyrolysis or by chemical reaction with HgO (s) or NHsfg). Trapping this gaseous species leads to the ring dimer dibromofuroxan. The BrCNO and (BrCNOh species are characterized in the gas phase by Hel photoelectron spectroscopy, Hel and HLa photoionization mass spectroscopy, and infrared spectroscopy. The geometry of BrCNO obtained from ab initio calculations at the HF, MP „(= 2—4), QCISD (T), and CCSD (T) levels of theory suggests a quasi-linear structure with a pronounced effect of triple substitutions on the linear—bent question. Thedibromofuroxan species, also investigated for the first time by gas-phase methods, is confirmed by ab initio calculations at the HF level as a planar five-membered ring with a strongly polarized exocyclic nitrile oxide group. This dimer, under …