The equilibrium structures, dipole moments, vibrational spectra, and relative energetics of the CINO3 isomers (ClONO2, ClOONO, OClONO, OClNO2) have been examined using various ab initio and density functional (MP2, CCSD(T), B3LYP) methods. The CIONO2 is found to be the lowest energy structure among the isomers. Cis-perp and trans-perp ClOONO are less stable by 25.3 and 26.7 kcal mol(-1), respectively. The OClONO isomeric form with the trans-perp conformation is 13.5 kcal mol(-1) higher in energy than the trans-perp ClOONO form. An examination of the vibrational and electronic spectra reveals spectral features that allow these species to be distinguished. We have considered the energetics pathways of ClOx/NOx reactions that may lead to the formation or dissociation of ClNO3 isomers.