The valence ionic states of ozone have been studied using a combination of multiconfiguration self-consistent field theory and configuration mixing techniques. It is found that the lower energy states of ozone show a significant mixing of one electron (primary) ionization and two electron simultaneous ionization-excitation processes leading to the predicted appearance of satellite bands with considerable intensity in the ultraviolet photoelectron spectrum. It is thus not possible to interpret the valence photoelectron spectrum of ozone in terms of the usual one electron orbital ionization processes. The calculations show that the same excited state configurations that are important in the neutral ozone optical absorption electronic spectrum also figure prominently as interacting excitation or “shake-up” states. The relationship between the relative degree of inadequacy of the frozen orbital approximation (Koopmans' theorem) when comparing ionic states energies and the importance of ionization-excitation processes is pointed out.