The unstable chloronitrile oxide molecule ClCNO has been generated from dichloroformaldoxime Cl2C=NOH by low-pressure gas-phase thermolysis or by a gas-solid reaction with HgO or metals. ClCNO is characterized in the gas phase by mid-infrared and HeI photoelectron spectroscopy, with the ground-state geometry obtained from ab initio calculations at the B3-LYP, MPn (n = 2-4), QCISD, QCISD(T), CCSD, and CCSD(T) levels using basis sets ranging from 6-31G** : through 6-311G(2d) to cc-pVTZ. The calculated structure is strongly dependent upon the method employed and on basis set size, but is predicted to be linear or quasi-linear. The stable ring dimer, dichlorofuroxan, separately synthesized, does not yield the monomer upon thermolysis but leads to ClC(O)CN as a major product. The ring dimer has been investigated in the gas phase by infrared, photoelectron, and photoionization mass spectroscopy and also by ab initio calculations at the B3-LYP/6-31G** level. The same level of theory was applied to an investigation of the mechanism by which ClCNO dimerizes, this being found to be a typical two-step process; the first being formation of the C-C bond and the second the formation of the N-O bond with ring closure.