Isomers of IO2, I2O, and I2O2 have been characterized by ab initio methods. Vibrational frequencies and geometries were obtained at the MP2/6-31G(d) and MP2/6-311+G(3df) levels of theory, respectively. The thermochemistry was derived using approximate QCISD(T)/G-311+G(3df) (Gaussian-2) energies coupled with isodesmic reactions. Computed values of Delta(f)H(298) for these molecules are the following (in kJ mol(-1)) : OIO, 76.7; IOO, 96.6; IIO, 134.1; IOI, 92.4; IOOI, 156.8; IIO2, 103.0; IOIO, 124.2; OIIO, 224.0. Uncertainties are discussed in the text. The results are compared with some previous empirical estimates and are employed in a QRRK analysis to interpret the IO + IO reaction : favorable products include I + OIO.