Ab initio calculations of vertical excitations to singlet and triplet excited states of chlorine nitrate and nitric acid are reported, using the CIS, CIS(D), and CCSD methods. The effects of basis set composition and calculational methods are investigated. The nature of the electronic transitions are examined by decomposing the difference density into the sum of detachment and attachment densities. Counterparts for the three lowest singlet excited states of nitric acid survive relatively unperturbed in chlorine nitrate, while other low lying singlet states of chlorine nitrate appear to be directly dissociative in the CIO chromophore. These results suggest an assignment of the two main peaks in the experimental chlorine nitrate absorption spectrum. In addition, triplet vertical excitations and the lowest optimized triplet geometries of both molecules are studied.