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 NH3(g). Trapping this gaseous species leads to the ring dimer dibromofuroxan. The BrCNO and (BrCNO)(2) species are characterized in the gas phase by HeI photoelectron spectroscopy, HeI and HL(alpha) photoionization mass spectroscopy, and infrared spectroscopy. The geometry of BrCNO obtained from ab initio calculations at the HF, MP(n) (n 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. The dibromofuroxan 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 thermolysis, does not revert to the BrCNO monomer, but leads to the formation of the unique fulminate, cyanogen bis-N-oxide, ONCCNO. The ab initio calculations lend strong support to the vibrational spectra, but are limited by the application of Koopmans’ theorem for the experimental ionization potentials.