Photochemical models of Io's atmosphere are presented with the assumption that SO2 is the major gas and that the SO2 surface pressure is controlled by vapor-pressure equilibrium at the surface. Photolysis of SO2 leads to efficient production of SO, O2, S, and O. Of these products, O and S are likely to be the dominant constituents in the upper atmosphere, and the atmospheric escape is expected to be in atomic form. Nonthermal escape processes are necessary to populate the plasma torus. SO2 surface densities greater than 1011 cm-3 are needed on the dayside to supply the required amounts of S and O to maintain the torus. Surface chemistry presents an interesting possibility, namely, the conversion of O to O2, in which case O2 will be the dominant constituents throughout the nightside and over the polar caps with a surface O2 density of 1010 cm-3.