The structures and energetics of the dichlorine oxides, Cl2On, n = 1-7, and their cations have been investigated theoretically at the Gaussian-2 (G2) and the Gaussian-3 (G3) levels of theory. The G2 and G3 heats of formation obtained for both neutral and cationic species allow the interpretation of assigned appearance energies and ionization energies reported in a recent photoionization mass spectrometric study (Ruhl et al. int. J. Mass Spectrom. 1999, 185, 545) on these chlorine oxides. The calculations predict that Cl2O6+ dissociates spontaneously to ClO2+ and ClO4, in agreement with the nonobservation of Cl2O6+ in the experimental study by Ruhl et al. For systems with n less than or equal to 4, the calculated and experimental thermochemical properties (such as heats of formation and ionization energies) are in good to excellent agreement with each other. However, this agreement deteriorates as the size of the molecular system increases or as n exceeds 4. Thus it appears that the G3 method may also suffer from "an unfavorable accumulation of component small errors," as has been found for the G2 method and its variants.