The molecular structures and energetics of ClOO, ClOOO, and ClO3 (C-3v) have been studied using four independent density functional theory methods. The methods used have been carefully calibrated against a comprehensive tabulation of experimental electron affinities (Chem. Rev. 2002, 102, 231). Optimized geometries, adiabatic electron affinities, vertical electron affinities, and vertical detachment energies are reported. Vibrational frequencies and bond dissociation energies are also reported, These density functional theory methods were utilized in conjunction with the double-zeta plus polarization basis set augmented with diffuse s-type and p-type functions (DZP++). Among these, the unrestricted BHLYP and 133LYP methods used for the open-shell ClOO and ClOOO systems lead to serious spin contamination, while the BLYP and BP86 methods have relatively little spin contamination ( < 0.76). The reliable predicted adiabatic electron affinities with the BLYP functional are 3.33 (ClOO), 3.97 (ClO3), and 2.84 (ClOOO) eV. The electronic ground state of ClOO- is a triplet ((3)A") state. There are seven local minima on the potential energy surface for ClOOO-; among them the gauche structure is the global minimum. The ClOO- and ClOOO- ground states may be regarded as Cl-...O2 and Cl-...O3 complexes, respectively. The ClOO- anion triplet state is predicted to lie 49-52 kcal/mol below OClO-. The neutral ClOOO isomer is predicted to lie 57 kcal/mol below the C-3nu structure ClO3. gauche-ClOOO- is predicted to lie 31 kcal/mol (BLYP) below the ClO3- (C-3nu) structure.