Abstract Quantum chemical calculations at various levels of theory have been performed in order to derive spectroscopically relevant parameters for the isomers of Cl 2O 2: chloryl chloride, ClClO 2, dichlorine peroxide, ClOOCl, and chlorine chlorite, ClOClO. For ClClO 2 an extented comparison with experimental data was possible. It was concluded that density functional theory (DFT) calculations using hybrid functionals (B3PW91, B3LYP) with various basis sets reproduced the experimental data quite well. The same is true for ClOClO (for which much less experimental data are available, however), but not for the peroxide, ClOOCl. Here extented ab initio calculations yielded better agreement, but peroxides, in general, seem to cause substantial problems for theoretical modeling.