Halogen dioxides (FOO, ClOO, BrOO, OClO, OBrO), their cationic and anionic derivatives and two isomers of ClO3 have been studied by means of density-functional theory (DFT) and the results compared with those from high level ab initio molecular orbital calculations. Three different density functionals (SVWN, B3LYP, and G96LYP) combined with a 6-311+G(2df ) basis set were used to obtain geometries and vibrational frequencies, which were then compared with MP2 (second-order Moller-Plesset), QCISD, and CCSD(T) (coupled-cluster single double triple) results. The B3LYP/6-311+G(2df ) calculations generally give geometries and frequencies in excellent agreement with those calculated from high level ab initio calculations such as CCSD(T). Exceptions, such as ClOO and BrOO, arise when high spin contamination at B3LYP level produces spurious results. Atomisation enthalpies evaluated at B3LYP/6-311+G(3df ) level of theory are observed to be in good agreement with the experimental values. In some particular cases this agreement is better than that obtained at CCSD(T)/6-311+G(3df ) level. For ionization enthalpies the CCSD(T) calculations seem to be superior to the DFT ones. Wave function instabilities [with respect to the UHF (unrestricted Hartree-Fock) transformation in the case of the cations and internal symmetry breaking in the case of the OXO (X=Cl, Br) compounds and the C-3v isomer of ClO3] are observed less frequently when DFT methods are used. (C) 2000 American Institute of Physics. [S0021-9606(00)30814-5].