Ab initio molecular orbital calculations have been carried out to determine the relative thermochemical stabilities and infrared vibrational spectra of isomers of the ClO dimer. Two straight-chain isomers, ClOOCl and ClOClO, and one branched species, ClCLO2, have been identified as energy minima on the Cl2O2 potential surface. The highest level calculations show ClOOCL and ClClO2 to be of comparable stability, with ClOClO some 11 kcal mol-1 higher in energy. Intense infrared absorptions for ClOOCl are calculated at 765,679, and 633 cm-1. These correlate closely with two broad bands (centered at 740 and 653 cm-1) observed by Molina and Molina (1987) to result from the gas-phase reactions of chlorine atoms with various ClO˙ precursors. The previous assignment of the 653 cm-1 band to ClOClO [Molina and Molina, 1987> is inconsistent with its calculated infrared spectrum. The calculations also suggest that the two most intense bands in the experimental infrared spectrum, at 1225 and 1057 cm-1 and previously assigned to ClOOCl, correspond instead to antisymmetric and symmetric streching modes of ClO2 functionality. These bands may indicate the presence of the third Cl2O2 isomer, ClClO2, or an alternative species such as ClOClO2.