The ionization energies of chlorine oxide (ClO) and its dimer (Cl2O2) have been measured using monochromatic synchrotron radiation in the 10-20 eV energy regime in combination with photoionization mass spectrometry. The threshold energy of ClO+ (m/z = 51) is found at 10.85 +/- 0.05 eV, whereas the ionization threshold of Cl2O2+ (m/z = 102) occurs at 11.05 +/- 0.05 eV. The experimental values are compared to results from ab initio calculations, where three stable isomers of Cl2O2 are considered : dichlorine peroxide (ClOOCl), chloryl chloride (ClClO2), and chlorine chlorite (ClOClO). The results indicate that the experimental threshold energy of Cl2O2+ is due to adiabatic ionization of ClOOCl. The heat of formation of ClOOCl+ is derived from the ionization threshold : Delta(f)H(298)(0)(ClOOCl+) = 1203 +/- 12 kJ/mol. Evidence for the occurrence of less stable Cl2O2 isomers, such as ClClO2 and ClOClO, is obtained from appearance energy measurements of the OClO+ (m/z = 67) fragment cation. The shift in ionization energy of ClOOCl relative to ClO is compared to other dimers of radicals, such as HO and NO2.