Unattenuated ionization rates for the dissociative photoionization of SO2 producing SO2+, SO+, S+, O+, and O2+ ions are reported during solar minimum and maximum conditions. It is found that in the atmosphere of Io the photo-dissociation-ionization of SO2 could be an important source of the production of SO+, S+, O+, and O2+ ions and that its contribution is much greater (factor of 5--10) compared with that due to direct photoionization of, respectively, SO, S, O, and O2 gases. Role of photoelectron impact ionization of SO2 in producing SO2+, SO+, S+, O+, and O2+ ions in the Io's atmosphere is investigated using a Monte Carlo model. It is recommended that the models of Io's ionosphere-atmosphere should include the dissociative ionization of SO2.¿ 1997 American Geophysical Union